Re: [postgis-users] Dot Density idea
On Fri, May 27, 2011 at 10:53:58AM -0600, John Abraham wrote:
> create or replace view dot_cnt as
> select the_geom, polygon_id, (quantity_column/100)::integer as numpoints
> from original_data;
>
> select dot_density('dot_cnt',
> 'the_geom',
> 'polygon_id',
> 'numpoints');
>
> alter table dp rename to save_it_for_future_use;
>
> And here are the functions:
It'd be nice if you could make your function accessible on the postgis
wiki, so it's easier to find it.
--strk;
() Free GIS & Flash consultant/developer
/\ http://strk.keybit.net/services.html
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Re: [postgis-users] Dot Density idea
Just to follow up on this, here is the code we (Abdel-Rahman M. Muhsen and I)
finally got around to writing. We found a function called randompoint which
just creates random points within the bounding box of the geometry from
Alexandre Sorokine.
http://sorokine.blogspot.com/2011/05/postgis-function-for-random-point.html (I
see he's updated it 4 days ago to be better for multi-polygons! We don't have
his updates yet.)
Then we wrote a function called dot_density which creates a table called dp
which has a point geometry for each point in the polygon. If there are
negative numbers in the point count, it creates the positive number of points
but flags them in another column. (We often plot dot density maps with red dots
for decreases and blue dots for increases.)
There are still a few improvements we'll likely make. We'd like to create the
geometry column using the addgeometrycolumn function. We'd like to specify the
output table name as a function parameter. There was also a good idea to use
Halton sequences or other "pseudo-random" sequences, instead of truly random
points, and Martin Davis implemented a few of the other "pseudo-random" ideas
in JTS
http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html .
Other suggestions are welcomed.
This is how we typically use it:
create or replace view dot_cnt as
select the_geom, polygon_id, (quantity_column/100)::integer as numpoints
from original_data;
select dot_density('dot_cnt',
'the_geom',
'polygon_id',
'numpoints');
alter table dp rename to save_it_for_future_use;
And here are the functions:
CREATE OR REPLACE FUNCTION randompoint(geom geometry)
RETURNS geometry AS
-- from Alexandre Sorokine
$BODY$
DECLARE
maxiter INTEGER := 1000;
i INTEGER := 0;
x0 DOUBLE PRECISION;
dx DOUBLE PRECISION;
y0 DOUBLE PRECISION;
dy DOUBLE PRECISION;
xp DOUBLE PRECISION;
yp DOUBLE PRECISION;
rpoint Geometry;
BEGIN
-- find envelope
x0 = ST_XMin(geom);
dx = (ST_XMax(geom) - x0);
y0 = ST_YMin(geom);
dy = (ST_YMax(geom) - y0);
WHILE i < maxiter LOOP
i = i + 1;
xp = x0 + dx * random();
yp = y0 + dy * random();
rpoint = ST_SetSRID( ST_MakePoint( xp, yp ), ST_SRID(geom) );
EXIT WHEN ST_Within( rpoint, geom );
END LOOP;
IF i > maxiter THEN
RAISE NOTICE 'number of interations exceeded max';
END IF;
RETURN rpoint;
END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE
COST 100;
ALTER FUNCTION randompoint(geometry) OWNER TO "usrPostgres";
-- Function: dot_density(text, text, text, text)
CREATE OR REPLACE FUNCTION dot_density(geom_table text, geom_col text, zone_col
text, num_of_points_col text)
RETURNS SETOF record AS
$BODY$
DECLARE
counter integer:=0;
tazrec record;
pointrec record;
result record;
num_points integer:=0;
np integer :=0;
begin
DROP sequence if exists randpnt_id;
CREATE sequence randpnt_id;
DROP TABLE IF EXISTS dp;
CREATE TABLE dp
(
gid integer PRIMARY KEY,
ser integer,
"zone" integer,
decrease_or_increase integer,
the_geom geometry
);
for tazrec in EXECUTE 'SELECT ' || zone_col || ' as geom_col , ' || zone_col ||
' as zone_col, '|| num_of_points_col || ' as num_of_points_col FROM ' ||
geom_table Loop
RAISE INFO 'Treating zone: %' , tazrec.zone_col;
num_points = tazrec.num_of_points_col;
IF num_points !=0 THEN np := num_points/abs(num_points);
ELSE np=0;
END IF;
EXECUTE 'INSERT INTO dp SELECT nextval(''randpnt_id'') as gid,
generate_series, '|| tazrec.zone_col || ', ' || np ||' , randompoint(the_geom)
FROM ' || geom_table || ', generate_series(1, '|| abs(num_points) ||') WHERE
'|| zone_col || '='|| tazrec.zone_col ;
END LOOP;
RETURN ;
end;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100
ROWS 1000;
ALTER FUNCTION dot_density(text, text, text, text) OWNER TO postgres;
On 2010-05-06, at 3:10 AM, strk wrote:
> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting
> function indeed. Sounds like a good job for GEOS/JTS.
>
> --strk;
>
> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote:
>> One of the things I miss about using ESRI's GIS is the ability to do
>> dot-density maps. Within a polygon, the number of dots is proportional to a
>> value, and the dots are randomly placed. I find it useful to be able to
>> present several data values at once (e.g. blue dots for population, red dots
>> for employment).
>>
>> I also find that it is a more intuitive way of scaling for zone size than
>> dividing the value by the area of the zone. That is, the count of the dots
>> represents the actual number, but the density of the dots represents the
>> density of the number. So I don't have to decide whether to divide the
>> value by the area of the polygon to plot density: both the absolute number
>> and the density are easily visible.
>>
>> Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't
>> plot dot-density, I've done without.
>>
>> But today I realized that I can build these on the
Re: [postgis-users] Dot Density idea
That's a neat idea alright. I'm not sure that calling it a "repulsive dots pattern" gets the right idea across though... 8^) Francis Markham wrote: Looks great Martin. Another way of generating non-clumpy random dots without a grid-like pattern is to use a "repulsive" function. See for example, http://www.statisticsblog.com/2010/06/repulsive-dots-pattern-the-difference-of-distance/ Cheers, Francis On 7 May 2010 08:11, Martin Davis wrote: Check out this blog post for some images of different kinds of random point fields: http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html Martin Davis wrote: Sounds like it could work - with maybe a bit of fiddling to deal with cases where the grid cells overlapped the polygon only slightly? Random perturbation by cell radius can still result in some points being very close together. (And I think this would also be an issue where only a small part of each grid cell overlapped the polygon). This may or may not be desirable. Perhaps a further check could be made to reduce the radius for points where this occurs. Or maybe some sort of simulated annealing process could be use to push the points into a more even distribution. M Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone.
Re: [postgis-users] Dot Density idea
Looks great Martin. Another way of generating non-clumpy random dots without a grid-like pattern is to use a "repulsive" function. See for example, http://www.statisticsblog.com/2010/06/repulsive-dots-pattern-the-difference-of-distance/ Cheers, Francis On 7 May 2010 08:11, Martin Davis wrote: > Check out this blog post for some images of different kinds of random point > fields: > > > http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html > > > Martin Davis wrote: > >> Sounds like it could work - with maybe a bit of fiddling to deal with >> cases where the grid cells overlapped the polygon only slightly? >> Random perturbation by cell radius can still result in some points being >> very close together. (And I think this would also be an issue where only a >> small part of each grid cell overlapped the polygon). This may or may not >> be desirable. Perhaps a further check could be made to reduce the radius >> for points where this occurs. Or maybe some sort of simulated annealing >> process could be use to push the points into a more even distribution. >> >> M >> >> Paul Ramsey wrote: >> >>> Even-yet-random :) nice requirement. How about just starting with a >>> regular grid and then perturbing the elements randomly with a radius >>> of a cell size? You can use the area of the polygon and number of >>> needed points to calculate the appropriate cell size and go from >>> there. >>> >>> P >>> >>> On Thu, May 6, 2010 at 10:28 AM, Martin Davis >>> wrote: >>> >>> >>>> Good point about the need for even distribution of the points. That >>>> seems >>>> like a whole lot harder to code than simply randomly placing points in a >>>> polygon. Does anyone have any pointers to algorithms for producing this >>>> effect? >>>> >>>> George Silva wrote: >>>> >>>> >>>>> The really big problem with dot density is that dots can overlap >>>>> themselves, >>>>> masking the real number, so if anything will be developed in this area, >>>>> the >>>>> points should be >>>>> >>>>> A) evenly distributed >>>>> or >>>>> B) randomly distributed, but with some sort of "colision" tests, so >>>>> there >>>>> is >>>>> no or little overlap. >>>>> >>>>> This is a interesting idea, especially if we could make a materialized >>>>> view >>>>> with those points, which could be added to GIS software for >>>>> presentation. >>>>> >>>>> George >>>>> >>>>> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < >>>>> ssuffic...@rov.sbcounty.gov> wrote: >>>>> >>>>> >>>>> >>>>> >>>>>> Looks nasty, but it might work: >>>>>> >>>>>> select >>>>>> st_line_interpolate_point( >>>>>> st_intersection( >>>>>> the_geom, >>>>>> st_makeline( >>>>>> st_pointn(st_exteriorring(the_geom), (rand1.rand >>>>>> * >>>>>> st_npoints(st_exteriorring(the_geom)))::int), >>>>>> st_pointn(st_exteriorring(the_geom), (rand2.rand >>>>>> * >>>>>> st_npoints(st_exteriorring(the_geom)))::int) >>>>>> ) >>>>>> ) >>>>>> ,rand3.rand >>>>>> ) >>>>>> from insert_your_table_name_here, >>>>>> (select random() as rand, generate_series(1,1000) as point_number) as >>>>>> rand1 >>>>>> JOIN (select random() as rand, generate_series(1,1000) as >>>>>> point_number) >>>>>> as >>>>>> rand2 >>>>>> ON rand1.point_number = rand2.point_number >>>>>> JOIN (select random() as rand, generate_series(1,1000) as >>>>>> point_number) >>>>>> as >>>>>> rand3 >>>>>> ON rand2.point_number = rand3.point_number >>>>>> WHERE st_geometrytype( >>>>>> st_intersection( >>>>>> the_geom, >>>>>> st_makeline( >>>>>>
Re: [postgis-users] Dot Density idea
gt;>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>>> Looks nasty, but it might work: >>>>>>>> >>>>>>>> select >>>>>>>> st_line_interpolate_point( >>>>>>>> st_intersection( >>>>>>>> the_geom, >>>>>>>> st_makeline( >>>>>>>> st_pointn(st_exteriorring(the_geom), >>>>>>>> (rand1.rand * >>>>>>>> st_npoints(st_exteriorring(the_geom)))::int), >>>>>>>> st_pointn(st_exteriorring(the_geom), >>>>>>>> (rand2.rand * >>>>>>>> st_npoints(st_exteriorring(the_geom)))::int) >>>>>>>> ) >>>>>>>> ) >>>>>>>> ,rand3.rand >>>>>>>> ) >>>>>>>> from insert_your_table_name_here, >>>>>>>> (select random() as rand, generate_series(1,1000) as point_number) >>>>>>>> as >>>>>>>> rand1 >>>>>>>> JOIN (select random() as rand, generate_series(1,1000) as >>>>>>>> point_number) >>>>>>>> as >>>>>>>> rand2 >>>>>>>> ON rand1.point_number = rand2.point_number >>>>>>>> JOIN (select random() as rand, generate_series(1,1000) as >>>>>>>> point_number) >>>>>>>> as >>>>>>>> rand3 >>>>>>>> ON rand2.point_number = rand3.point_number >>>>>>>> WHERE st_geometrytype( >>>>>>>> st_intersection( >>>>>>>> the_geom, >>>>>>>> st_makeline( >>>>>>>> st_pointn(st_exteriorring(the_geom), >>>>>>>> (rand1.rand * >>>>>>>> st_npoints(st_exteriorring(the_geom)))::int), >>>>>>>> st_pointn(st_exteriorring(the_geom), >>>>>>>> (rand2.rand * >>>>>>>> st_npoints(st_exteriorring(the_geom)))::int) >>>>>>>> ) >>>>>>>> ) >>>>>>>> ) = 'ST_LineString' >>>>>>>> AND oid = 5030 /* Enter your own OID here */ >>>>>>>> limit 100 >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> -Original Message- >>>>>>>>> From: postgis-users-boun...@postgis.refractions.net >>>>>>>>> [mailto:postgis-users-boun...@postgis.refractions.net] On >>>>>>>>> Behalf Of Martin Davis >>>>>>>>> Sent: Thursday, May 06, 2010 8:56 AM >>>>>>>>> To: John Abraham; postgis-users@postgis.refractions.net; Martin >>>>>>>>> Davis >>>>>>>>> Subject: Re: [postgis-users] Dot Density idea >>>>>>>>> >>>>>>>>> >>>>>>>>> I was thinking the same thing! >>>>>>>>> >>>>>>>>> strk wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> ST_RandomPoinsOnSurface(geometry, numpoints) would be an >>>>>>>>>> interesting >>>>>>>>>> function indeed. Sounds like a good job for GEOS/JTS. >>>>>>>>>> >>>>>>>>>> --strk; >>>>>>>>>> >>>>>>>>>> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> One of the things I miss about using ESRI's GIS is the >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>
Re: [postgis-users] Dot Density idea
How about this site? http://www.johndcook.com/blog/2009/03/16/quasi-random-sequences-in-art-and-integration/ Jan On 05/07/10 03:59, Ben Madin wrote: Martin, Assuming you wanted feedback - from an epidemiologists perspective, I like the first one - Complete Spatial Randomness is an important concept to avoid engendering a perception of order or clustering, and I'm thinking as long as the relative 'dot-density' is correct for each polygon, the CSR approach is less like to lead people to think the dots are located on significant points - in the context of obscuring actual locations etc this is important. If you want it more regular, then maybe you don't need to randomise it - or maybe this just means the function needs a user parameter to be able to set the level of randomisation - From 0 (completely ordered) to 1 (completely Random) (... to 2.5 completely clustered?) Just my thoughts - also for my money, I wouldn't change dot sizes. very confusing. cheers Ben On 07/05/2010, at 6:11 , Martin Davis wrote: Check out this blog post for some images of different kinds of random point fields: http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html Martin Davis wrote: Sounds like it could work - with maybe a bit of fiddling to deal with cases where the grid cells overlapped the polygon only slightly? Random perturbation by cell radius can still result in some points being very close together. (And I think this would also be an issue where only a small part of each grid cell overlapped the polygon). This may or may not be desirable. Perhaps a further check could be made to reduce the radius for points where this occurs. Or maybe some sort of simulated annealing process could be use to push the points into a more even distribution. M Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley< ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the
Re: [postgis-users] Dot Density idea
Martin, Assuming you wanted feedback - from an epidemiologists perspective, I like the first one - Complete Spatial Randomness is an important concept to avoid engendering a perception of order or clustering, and I'm thinking as long as the relative 'dot-density' is correct for each polygon, the CSR approach is less like to lead people to think the dots are located on significant points - in the context of obscuring actual locations etc this is important. If you want it more regular, then maybe you don't need to randomise it - or maybe this just means the function needs a user parameter to be able to set the level of randomisation - From 0 (completely ordered) to 1 (completely Random) (... to 2.5 completely clustered?) Just my thoughts - also for my money, I wouldn't change dot sizes. very confusing. cheers Ben On 07/05/2010, at 6:11 , Martin Davis wrote: > Check out this blog post for some images of different kinds of random point > fields: > > http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html > > Martin Davis wrote: >> Sounds like it could work - with maybe a bit of fiddling to deal with cases >> where the grid cells overlapped the polygon only slightly? >> Random perturbation by cell radius can still result in some points being >> very close together. (And I think this would also be an issue where only a >> small part of each grid cell overlapped the polygon). This may or may not >> be desirable. Perhaps a further check could be made to reduce the radius >> for points where this occurs. Or maybe some sort of simulated annealing >> process could be use to push the points into a more even distribution. >> >> M >> >> Paul Ramsey wrote: >>> Even-yet-random :) nice requirement. How about just starting with a >>> regular grid and then perturbing the elements randomly with a radius >>> of a cell size? You can use the area of the polygon and number of >>> needed points to calculate the appropriate cell size and go from >>> there. >>> >>> P >>> >>> On Thu, May 6, 2010 at 10:28 AM, Martin Davis >>> wrote: >>> >>>> Good point about the need for even distribution of the points. That seems >>>> like a whole lot harder to code than simply randomly placing points in a >>>> polygon. Does anyone have any pointers to algorithms for producing this >>>> effect? >>>> >>>> George Silva wrote: >>>> >>>>> The really big problem with dot density is that dots can overlap >>>>> themselves, >>>>> masking the real number, so if anything will be developed in this area, >>>>> the >>>>> points should be >>>>> >>>>> A) evenly distributed >>>>> or >>>>> B) randomly distributed, but with some sort of "colision" tests, so there >>>>> is >>>>> no or little overlap. >>>>> >>>>> This is a interesting idea, especially if we could make a materialized >>>>> view >>>>> with those points, which could be added to GIS software for presentation. >>>>> >>>>> George >>>>> >>>>> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < >>>>> ssuffic...@rov.sbcounty.gov> wrote: >>>>> >>>>> >>>>> >>>>>> Looks nasty, but it might work: >>>>>> >>>>>> select >>>>>> st_line_interpolate_point( >>>>>> st_intersection( >>>>>> the_geom, >>>>>> st_makeline( >>>>>> st_pointn(st_exteriorring(the_geom), (rand1.rand * >>>>>> st_npoints(st_exteriorring(the_geom)))::int), >>>>>> st_pointn(st_exteriorring(the_geom), (rand2.rand * >>>>>> st_npoints(st_exteriorring(the_geom)))::int) >>>>>> ) >>>>>> ) >>>>>> ,rand3.rand >>>>>> ) >>>>>> from insert_your_table_name_here, >>>>>> (select random() as rand, generate_series(1,1000) as point_number) as >>>>>> rand1 >>>>>> JOIN (select random() as rand, generate_series(1,1000) as point_number) >>>>>> as >>>>>> rand2 >>>>>> ON rand1.point_number = rand2.point_number >>>>>> JOIN (se
Re: [postgis-users] Dot Density idea
Check out this blog post for some images of different kinds of random point fields: http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html Martin Davis wrote: Sounds like it could work - with maybe a bit of fiddling to deal with cases where the grid cells overlapped the polygon only slightly? Random perturbation by cell radius can still result in some points being very close together. (And I think this would also be an issue where only a small part of each grid cell overlapped the polygon). This may or may not be desirable. Perhaps a further check could be made to reduce the radius for points where this occurs. Or maybe some sort of simulated annealing process could be use to push the points into a more even distribution. M Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible. Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without. But today I realized that I can build these on the server instead. I can generate random points within
Re: [postgis-users] Dot Density idea
Which is a nice segue into this thread: http://sourceforge.net/mailarchive/forum.php?thread_name=s2he24c4c271005042228wfff7b9abjadf3eb357fe0c712%40mail.gmail.com&forum_name=jts-topo-suite-user Norman Vine wrote: When I have done this before I usually triangulate the polygon first then use each triangles area to normalize the number of points randomly inserted into each triangle On May 6, 2010, at 1:51 PM, David William Bitner wrote: If it was the overall density of the dots that mattered and not as much the exact number of dots, a representative table could be made with a "random yet even" set of dots covering a certain area. To apply to a polygon, you could then Scale the data to the appropriate density and translate/compute the intersection to apply to a given polygon. On Thu, May 6, 2010 at 12:50 PM, Martin Davis wrote: Did your algorithm work with irregular polgon extents? Jan Hartmann wrote: I did this long ago. Regular points won't work, you'll get all kinds of moire patterns. I just computed random x and y values from the origin of the grid cell within the x- and y grid-size. Can't remember having any problems with overlapping points. Jan On 05/06/10 19:35, Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley< ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to
Re: [postgis-users] Dot Density idea
When I have done this before I usually triangulate the polygon first then use each triangles area to normalize the number of points randomly inserted into each triangle On May 6, 2010, at 1:51 PM, David William Bitner wrote: > If it was the overall density of the dots that mattered and not as much the > exact number of dots, a representative table could be made with a "random yet > even" set of dots covering a certain area. To apply to a polygon, you could > then Scale the data to the appropriate density and translate/compute the > intersection to apply to a given polygon. > > On Thu, May 6, 2010 at 12:50 PM, Martin Davis wrote: > Did your algorithm work with irregular polgon extents? > > > Jan Hartmann wrote: > I did this long ago. Regular points won't work, you'll get all kinds of moire > patterns. I just computed random x and y values from the origin of the grid > cell within the x- and y grid-size. Can't remember having any problems with > overlapping points. > > Jan > > On 05/06/10 19:35, Paul Ramsey wrote: > Even-yet-random :) nice requirement. How about just starting with a > regular grid and then perturbing the elements randomly with a radius > of a cell size? You can use the area of the polygon and number of > needed points to calculate the appropriate cell size and go from > there. > > P > > On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: > > Good point about the need for even distribution of the points. That seems > like a whole lot harder to code than simply randomly placing points in a > polygon. Does anyone have any pointers to algorithms for producing this > effect? > > George Silva wrote: > > The really big problem with dot density is that dots can overlap > themselves, > masking the real number, so if anything will be developed in this area, > the > points should be > > A) evenly distributed > or > B) randomly distributed, but with some sort of "colision" tests, so there > is > no or little overlap. > > This is a interesting idea, especially if we could make a materialized > view > with those points, which could be added to GIS software for presentation. > > George > > On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley< > ssuffic...@rov.sbcounty.gov> wrote: > > > > Looks nasty, but it might work: > > select > st_line_interpolate_point( > st_intersection( > the_geom, > st_makeline( > st_pointn(st_exteriorring(the_geom), (rand1.rand * > st_npoints(st_exteriorring(the_geom)))::int), > st_pointn(st_exteriorring(the_geom), (rand2.rand * > st_npoints(st_exteriorring(the_geom)))::int) > ) > ) > ,rand3.rand > ) > from insert_your_table_name_here, > (select random() as rand, generate_series(1,1000) as point_number) as > rand1 > JOIN (select random() as rand, generate_series(1,1000) as point_number) > as > rand2 > ON rand1.point_number = rand2.point_number > JOIN (select random() as rand, generate_series(1,1000) as point_number) > as > rand3 > ON rand2.point_number = rand3.point_number > WHERE st_geometrytype( > st_intersection( > the_geom, > st_makeline( > st_pointn(st_exteriorring(the_geom), (rand1.rand * > st_npoints(st_exteriorring(the_geom)))::int), > st_pointn(st_exteriorring(the_geom), (rand2.rand * > st_npoints(st_exteriorring(the_geom)))::int) > ) > ) > ) = 'ST_LineString' > AND oid = 5030 /* Enter your own OID here */ > limit 100 > > > > > > -Original Message- > From: postgis-users-boun...@postgis.refractions.net > [mailto:postgis-users-boun...@postgis.refractions.net] On > Behalf Of Martin Davis > Sent: Thursday, May 06, 2010 8:56 AM > To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis > Subject: Re: [postgis-users] Dot Density idea > > > I was thinking the same thing! > > strk wrote: > > > ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting > function indeed. Sounds like a good job for GEOS/JTS. > > --strk; > > On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: > > > > One of the things I miss about using ESRI's GIS is the > > > ability to do > > > dot-density maps. Within a polygon, the number of dots is > > > proportional to a value, and the dots are randomly placed. I > find it useful to be able to present several data values
Re: [postgis-users] Dot Density idea
Hi, We use an application much like this for generating random sample points for 2-phase random stratified trawl biomass surveys. Specify polygon name, number of points, minimum distance from the boundary, minimum distance between points in a file, then iterate through the file. The code has a limit on the number of iterations per stratum to try before deciding that we can't actually fit that many points in it with the current parameters. Strata can be polygons or multipolygons, with or without holes. If anyone is really interested, a paper describing the approach for such survey design & analysis is at: http://www.royalsociety.org.nz/Site/publish/Journals/nzjmfr/1984/8.aspx This simply makes the assumption that the distribution of sample sites is random. The application we use is in C++, but I wrote a PostGIS version a few years ago as a bash script to demonstrate the functionality from within a db. One of the issues we ran into was running repeated iterations then looking at the distribution of the points across iterations to check that there was genuinely no pattern. Generally most random functions we tested were not truly random, there was a trend for coordinates to lie towards the top right or bottom left, as random values tended to be < or > 0.5. Not that noticeable with a single iteration but statistically present. We wrote our own random() function in the program to get around such artifacts. Cheers, Brent Wood --- On Fri, 5/7/10, Martin Davis wrote: > From: Martin Davis > Subject: Re: [postgis-users] Dot Density idea > To: "PostGIS Users Discussion" > Date: Friday, May 7, 2010, 5:50 AM > Did your algorithm work with > irregular polgon extents? > > Jan Hartmann wrote: > > I did this long ago. Regular points won't work, you'll > get all kinds of moire patterns. I just computed random x > and y values from the origin of the grid cell within the x- > and y grid-size. Can't remember having any problems with > overlapping points. > > > > Jan > > > > On 05/06/10 19:35, Paul Ramsey wrote: > >> Even-yet-random :) nice requirement. How about > just starting with a > >> regular grid and then perturbing the elements > randomly with a radius > >> of a cell size? You can use the area of the > polygon and number of > >> needed points to calculate the appropriate cell > size and go from > >> there. > >> > >> P > >> > >> On Thu, May 6, 2010 at 10:28 AM, Martin > Davis > wrote: > >> > >>> Good point about the need for even > distribution of the points. That seems > >>> like a whole lot harder to code than simply > randomly placing points in a > >>> polygon. Does anyone have any pointers > to algorithms for producing this > >>> effect? > >>> > >>> George Silva wrote: > >>> > >>>> The really big problem with dot density is > that dots can overlap > >>>> themselves, > >>>> masking the real number, so if anything > will be developed in this area, > >>>> the > >>>> points should be > >>>> > >>>> A) evenly distributed > >>>> or > >>>> B) randomly distributed, but with some > sort of "colision" tests, so there > >>>> is > >>>> no or little overlap. > >>>> > >>>> This is a interesting idea, especially if > we could make a materialized > >>>> view > >>>> with those points, which could be added to > GIS software for presentation. > >>>> > >>>> George > >>>> > >>>> On Thu, May 6, 2010 at 1:53 PM, Sufficool, > Stanley< > >>>> ssuffic...@rov.sbcounty.gov> > wrote: > >>>> > >>>> > >>>> > >>>>> Looks nasty, but it might work: > >>>>> > >>>>> select > >>>>> st_line_interpolate_point( > >>>>> > st_intersection( > >>>>> > the_geom, > >>>>> > st_makeline( > >>>>> > > st_pointn(st_exteriorring(the_geom), (rand1.rand * > >>>>> > st_npoints(st_exteriorring(the_geom)))::int), > >>>>> > > st_pointn(st_exteriorring(the_geom), (rand2.rand * > >>>>> > st_npoints(st_exteriorring(the_geom)))::int) > >>>>> > ) > >&
Re: [postgis-users] Dot Density idea
Yea, this is very cool indeed. George On Thu, May 6, 2010 at 3:33 PM, David Fawcett wrote: > Doesn't OpenSource suck!!! > > > On Mon, May 3, 2010 at 11:49 PM, John Abraham > wrote: > > One of the things I miss about using ESRI's GIS is the ability to do > dot-density maps. Within a polygon, the number of dots is > > An then an hour later... > > > http://lin-ear-th-inking.blogspot.com/2010/05/random-points-in-polygon-in-jts.html > > Sure, it's not a PostGIS solution and isn't exactly there yet, but not > bad... > > Imagine the converse: > > Dear Jack, > > There is this useful feature that is missing from your software, when > do you think that you could get it into a release?... > ___ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > -- George R. C. Silva Desenvolvimento em GIS http://blog.geoprocessamento.net ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
Doesn't OpenSource suck!!! On Mon, May 3, 2010 at 11:49 PM, John Abraham wrote: > One of the things I miss about using ESRI's GIS is the ability to do > dot-density maps. Within a polygon, the number of dots is An then an hour later... http://lin-ear-th-inking.blogspot.com/2010/05/random-points-in-polygon-in-jts.html Sure, it's not a PostGIS solution and isn't exactly there yet, but not bad... Imagine the converse: Dear Jack, There is this useful feature that is missing from your software, when do you think that you could get it into a release?... ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
orring(the_geom), (rand1.rand *
st_npoints(st_exteriorring(the_geom)))::int),
st_pointn(st_exteriorring(the_geom), (rand2.rand *
st_npoints(st_exteriorring(the_geom)))::int)
)
)
) = 'ST_LineString'
AND oid = 5030 /* Enter your own OID here */
limit 100
-Original Message-
From:
postgis-users-boun...@postgis.refractions.net
<mailto:postgis-users-boun...@postgis.refractions.net>
[mailto:postgis-users-boun...@postgis.refractions.net
<mailto:postgis-users-boun...@postgis.refractions.net>]
On
Behalf Of Martin Davis
Sent: Thursday, May 06, 2010 8:56 AM
To: John Abraham;
postgis-users@postgis.refractions.net
<mailto:postgis-users@postgis.refractions.net>;
Martin Davis
Subject: Re: [postgis-users] Dot Density idea
I was thinking the same thing!
strk wrote:
ST_RandomPoinsOnSurface(geometry,
numpoints) would be an interesting
function indeed. Sounds like a good
job for GEOS/JTS.
--strk;
On Mon, May 03, 2010 at 10:49:32PM
-0600, John Abraham wrote:
One of the things I miss about
using ESRI's GIS is the
ability to do
dot-density maps. Within a
polygon, the number of dots is
proportional to a value, and the dots are
randomly placed. I
find it useful to be able to present
several data values at
once (e.g. blue dots for population, red
dots for employment).
I also find that it is a more
intuitive way of scaling for
zone size
than dividing the value by the
area of the zone. That is,
the count
of the dots represents the actual
number, but the density
of the dots
represents the density of the
number. So I don't have to decide
whether to divide the value by the
area of the polygon to plot
density: both the absolute number
and the density are
easily visible.
Since my open-source GIS viewing
systems (mostly QGIS and
Mapserver)
won't plot dot-density, I've done
without.
But today I realized that I can
build these on the server
instead. I
can generate random points within
the bounding-box of the polygon,
throwing out those that aren't
contained within the polygon,
repeating until I have enough.
Then I can save these points as a
separate layer, and display this
layer using almost any desktop or
web based viewer!
Has anyone done this? Can I do it
in SQL or do I need to write
something in PL/pgsql?
--
John Abraham
Re: [postgis-users] Dot Density idea
If it was the overall density of the dots that mattered and not as much the exact number of dots, a representative table could be made with a "random yet even" set of dots covering a certain area. To apply to a polygon, you could then Scale the data to the appropriate density and translate/compute the intersection to apply to a given polygon. On Thu, May 6, 2010 at 12:50 PM, Martin Davis wrote: > Did your algorithm work with irregular polgon extents? > > > Jan Hartmann wrote: > >> I did this long ago. Regular points won't work, you'll get all kinds of >> moire patterns. I just computed random x and y values from the origin of the >> grid cell within the x- and y grid-size. Can't remember having any problems >> with overlapping points. >> >> Jan >> >> On 05/06/10 19:35, Paul Ramsey wrote: >> >>> Even-yet-random :) nice requirement. How about just starting with a >>> regular grid and then perturbing the elements randomly with a radius >>> of a cell size? You can use the area of the polygon and number of >>> needed points to calculate the appropriate cell size and go from >>> there. >>> >>> P >>> >>> On Thu, May 6, 2010 at 10:28 AM, Martin Davis >>> wrote: >>> >>> >>>> Good point about the need for even distribution of the points. That >>>> seems >>>> like a whole lot harder to code than simply randomly placing points in a >>>> polygon. Does anyone have any pointers to algorithms for producing this >>>> effect? >>>> >>>> George Silva wrote: >>>> >>>> >>>>> The really big problem with dot density is that dots can overlap >>>>> themselves, >>>>> masking the real number, so if anything will be developed in this area, >>>>> the >>>>> points should be >>>>> >>>>> A) evenly distributed >>>>> or >>>>> B) randomly distributed, but with some sort of "colision" tests, so >>>>> there >>>>> is >>>>> no or little overlap. >>>>> >>>>> This is a interesting idea, especially if we could make a materialized >>>>> view >>>>> with those points, which could be added to GIS software for >>>>> presentation. >>>>> >>>>> George >>>>> >>>>> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley< >>>>> ssuffic...@rov.sbcounty.gov> wrote: >>>>> >>>>> >>>>> >>>>> >>>>>> Looks nasty, but it might work: >>>>>> >>>>>> select >>>>>> st_line_interpolate_point( >>>>>> st_intersection( >>>>>> the_geom, >>>>>> st_makeline( >>>>>> st_pointn(st_exteriorring(the_geom), (rand1.rand >>>>>> * >>>>>> st_npoints(st_exteriorring(the_geom)))::int), >>>>>> st_pointn(st_exteriorring(the_geom), (rand2.rand >>>>>> * >>>>>> st_npoints(st_exteriorring(the_geom)))::int) >>>>>> ) >>>>>> ) >>>>>> ,rand3.rand >>>>>> ) >>>>>> from insert_your_table_name_here, >>>>>> (select random() as rand, generate_series(1,1000) as point_number) as >>>>>> rand1 >>>>>> JOIN (select random() as rand, generate_series(1,1000) as >>>>>> point_number) >>>>>> as >>>>>> rand2 >>>>>> ON rand1.point_number = rand2.point_number >>>>>> JOIN (select random() as rand, generate_series(1,1000) as >>>>>> point_number) >>>>>> as >>>>>> rand3 >>>>>> ON rand2.point_number = rand3.point_number >>>>>> WHERE st_geometrytype( >>>>>> st_intersection( >>>>>> the_geom, >>>>>> st_makeline( >>>>>> st_pointn(st_exteriorring(the_geom), (rand1.rand >>>>>> * >>>>>> st_npoints(st_exteriorring(the_geom)))::int), >>>>>> st_pointn(st_exteriorring(the_geom), (rand2.rand >>>>>> * >>>>>>
Re: [postgis-users] Dot Density idea
Did your algorithm work with irregular polgon extents? Jan Hartmann wrote: I did this long ago. Regular points won't work, you'll get all kinds of moire patterns. I just computed random x and y values from the origin of the grid cell within the x- and y grid-size. Can't remember having any problems with overlapping points. Jan On 05/06/10 19:35, Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley< ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible. Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without. But today I realized that I can build these on the server instead. I can generate random points within the bounding-box of the polygon, throwing out those that aren't contained within the polygon, repeating until I have enough. Then I can save these points as a separate layer, and display this layer using almost any desktop or web based viewer! Has anyone done this? Can I do it in SQL or do I need to write something in PL/pgsql? -- John Abraha
Re: [postgis-users] Dot Density idea
Sounds like it could work - with maybe a bit of fiddling to deal with cases where the grid cells overlapped the polygon only slightly? Random perturbation by cell radius can still result in some points being very close together. (And I think this would also be an issue where only a small part of each grid cell overlapped the polygon). This may or may not be desirable. Perhaps a further check could be made to reduce the radius for points where this occurs. Or maybe some sort of simulated annealing process could be use to push the points into a more even distribution. M Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible. Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without. But today I realized that I can build these on the server instead. I can generate random points within the bounding-box of the polygon, throwing out those that aren't contained within the polygon, repeating until I have enough. Then I can save these points as a separate layer, and displ
Re: [postgis-users] Dot Density idea
I did this long ago. Regular points won't work, you'll get all kinds of moire patterns. I just computed random x and y values from the origin of the grid cell within the x- and y grid-size. Can't remember having any problems with overlapping points. Jan On 05/06/10 19:35, Paul Ramsey wrote: Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley< ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible. Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without. But today I realized that I can build these on the server instead. I can generate random points within the bounding-box of the polygon, throwing out those that aren't contained within the polygon, repeating until I have enough. Then I can save these points as a separate layer, and display this layer using almost any desktop or web based viewer! Has anyone done this? Can I do it in SQL or do I need to write something in PL/pgsql? -- John Abraham PS I just bought the Postgis In Action book; enjoying it so far. __
Re: [postgis-users] Dot Density idea
Even-yet-random :) nice requirement. How about just starting with a regular grid and then perturbing the elements randomly with a radius of a cell size? You can use the area of the polygon and number of needed points to calculate the appropriate cell size and go from there. P On Thu, May 6, 2010 at 10:28 AM, Martin Davis wrote: > Good point about the need for even distribution of the points. That seems > like a whole lot harder to code than simply randomly placing points in a > polygon. Does anyone have any pointers to algorithms for producing this > effect? > > George Silva wrote: >> >> The really big problem with dot density is that dots can overlap >> themselves, >> masking the real number, so if anything will be developed in this area, >> the >> points should be >> >> A) evenly distributed >> or >> B) randomly distributed, but with some sort of "colision" tests, so there >> is >> no or little overlap. >> >> This is a interesting idea, especially if we could make a materialized >> view >> with those points, which could be added to GIS software for presentation. >> >> George >> >> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < >> ssuffic...@rov.sbcounty.gov> wrote: >> >> >>> >>> Looks nasty, but it might work: >>> >>> select >>> st_line_interpolate_point( >>> st_intersection( >>> the_geom, >>> st_makeline( >>> st_pointn(st_exteriorring(the_geom), (rand1.rand * >>> st_npoints(st_exteriorring(the_geom)))::int), >>> st_pointn(st_exteriorring(the_geom), (rand2.rand * >>> st_npoints(st_exteriorring(the_geom)))::int) >>> ) >>> ) >>> ,rand3.rand >>> ) >>> from insert_your_table_name_here, >>> (select random() as rand, generate_series(1,1000) as point_number) as >>> rand1 >>> JOIN (select random() as rand, generate_series(1,1000) as point_number) >>> as >>> rand2 >>> ON rand1.point_number = rand2.point_number >>> JOIN (select random() as rand, generate_series(1,1000) as point_number) >>> as >>> rand3 >>> ON rand2.point_number = rand3.point_number >>> WHERE st_geometrytype( >>> st_intersection( >>> the_geom, >>> st_makeline( >>> st_pointn(st_exteriorring(the_geom), (rand1.rand * >>> st_npoints(st_exteriorring(the_geom)))::int), >>> st_pointn(st_exteriorring(the_geom), (rand2.rand * >>> st_npoints(st_exteriorring(the_geom)))::int) >>> ) >>> ) >>> ) = 'ST_LineString' >>> AND oid = 5030 /* Enter your own OID here */ >>> limit 100 >>> >>> >>> >>> >>>> >>>> -Original Message- >>>> From: postgis-users-boun...@postgis.refractions.net >>>> [mailto:postgis-users-boun...@postgis.refractions.net] On >>>> Behalf Of Martin Davis >>>> Sent: Thursday, May 06, 2010 8:56 AM >>>> To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis >>>> Subject: Re: [postgis-users] Dot Density idea >>>> >>>> >>>> I was thinking the same thing! >>>> >>>> strk wrote: >>>> >>>>> >>>>> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting >>>>> function indeed. Sounds like a good job for GEOS/JTS. >>>>> >>>>> --strk; >>>>> >>>>> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: >>>>> >>>>> >>>>>> >>>>>> One of the things I miss about using ESRI's GIS is the >>>>>> >>>> >>>> ability to do >>>> >>>>>> >>>>>> dot-density maps. Within a polygon, the number of dots is >>>>>> >>>> >>>> proportional to a value, and the dots are randomly placed. I >>>> find it useful to be able to present several data values at >>>> once (e.g. blue dots for population, red dots for employment). >>>> >>>>>> >>>>>> I also find that it is a more intuitive way of scaling for >>>>>> >>>> >>>> zone size >>>> >>>>>> >>>>&g
Re: [postgis-users] Dot Density idea
Using St_SnapToGrid is a way to do it. A simple algorithm could calculate the ideal distance between points depending on the number of resulting points and then just place them in correct place. The way to do it is to inform attribute and the value of each dot. On the same note, a proportional symbols representation could be done, but that can be very tricky, since building the actual circles could give you extensive overlap, and they should change according to scale, so I guess that could be done by software. PostGIS could generate a view using the centroid for proportional symbols using a point a softwares need to take care of the representation. George On Thu, May 6, 2010 at 2:28 PM, Martin Davis wrote: > Good point about the need for even distribution of the points. That seems > like a whole lot harder to code than simply randomly placing points in a > polygon. Does anyone have any pointers to algorithms for producing this > effect? > > George Silva wrote: > >> The really big problem with dot density is that dots can overlap >> themselves, >> masking the real number, so if anything will be developed in this area, >> the >> points should be >> >> A) evenly distributed >> or >> B) randomly distributed, but with some sort of "colision" tests, so there >> is >> no or little overlap. >> >> This is a interesting idea, especially if we could make a materialized >> view >> with those points, which could be added to GIS software for presentation. >> >> George >> >> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < >> ssuffic...@rov.sbcounty.gov> wrote: >> >> >> >>> Looks nasty, but it might work: >>> >>> select >>> st_line_interpolate_point( >>> st_intersection( >>> the_geom, >>> st_makeline( >>> st_pointn(st_exteriorring(the_geom), (rand1.rand * >>> st_npoints(st_exteriorring(the_geom)))::int), >>> st_pointn(st_exteriorring(the_geom), (rand2.rand * >>> st_npoints(st_exteriorring(the_geom)))::int) >>> ) >>> ) >>> ,rand3.rand >>> ) >>> from insert_your_table_name_here, >>> (select random() as rand, generate_series(1,1000) as point_number) as >>> rand1 >>> JOIN (select random() as rand, generate_series(1,1000) as point_number) >>> as >>> rand2 >>> ON rand1.point_number = rand2.point_number >>> JOIN (select random() as rand, generate_series(1,1000) as point_number) >>> as >>> rand3 >>> ON rand2.point_number = rand3.point_number >>> WHERE st_geometrytype( >>> st_intersection( >>> the_geom, >>> st_makeline( >>> st_pointn(st_exteriorring(the_geom), (rand1.rand * >>> st_npoints(st_exteriorring(the_geom)))::int), >>> st_pointn(st_exteriorring(the_geom), (rand2.rand * >>> st_npoints(st_exteriorring(the_geom)))::int) >>> ) >>> ) >>> ) = 'ST_LineString' >>> AND oid = 5030 /* Enter your own OID here */ >>> limit 100 >>> >>> >>> >>> >>> >>>> -Original Message- >>>> From: postgis-users-boun...@postgis.refractions.net >>>> [mailto:postgis-users-boun...@postgis.refractions.net] On >>>> Behalf Of Martin Davis >>>> Sent: Thursday, May 06, 2010 8:56 AM >>>> To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis >>>> Subject: Re: [postgis-users] Dot Density idea >>>> >>>> >>>> I was thinking the same thing! >>>> >>>> strk wrote: >>>> >>>> >>>>> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting >>>>> function indeed. Sounds like a good job for GEOS/JTS. >>>>> >>>>> --strk; >>>>> >>>>> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: >>>>> >>>>> >>>>> >>>>>> One of the things I miss about using ESRI's GIS is the >>>>>> >>>>>> >>>>> ability to do >>>> >>>> >>>>> dot-density maps. Within a polygon, the number of dots is >>>>>> >>>>>> >>>>> proportional to a value, and the dots are randomly placed. I >>>> find it usefu
Re: [postgis-users] Dot Density idea
Good point about the need for even distribution of the points. That seems like a whole lot harder to code than simply randomly placing points in a polygon. Does anyone have any pointers to algorithms for producing this effect? George Silva wrote: The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < ssuffic...@rov.sbcounty.gov> wrote: Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 -Original Message- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Martin Davis Sent: Thursday, May 06, 2010 8:56 AM To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis Subject: Re: [postgis-users] Dot Density idea I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible. Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without. But today I realized that I can build these on the server instead. I can generate random points within the bounding-box of the polygon, throwing out those that aren't contained within the polygon, repeating until I have enough. Then I can save these points as a separate layer, and display this layer using almost any desktop or web based viewer! Has anyone done this? Can I do it in SQL or do I need to write something in PL/pgsql? -- John Abraham PS I just bought the Postgis In Action book; enjoying it so far. ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users -- Martin Davis Senior Technical Architect Refractions Research, Inc. (250) 383-3022 ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users ___ postgis-users mail
Re: [postgis-users] Dot Density idea
The really big problem with dot density is that dots can overlap themselves, masking the real number, so if anything will be developed in this area, the points should be A) evenly distributed or B) randomly distributed, but with some sort of "colision" tests, so there is no or little overlap. This is a interesting idea, especially if we could make a materialized view with those points, which could be added to GIS software for presentation. George On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley < ssuffic...@rov.sbcounty.gov> wrote: > Looks nasty, but it might work: > > select > st_line_interpolate_point( >st_intersection( >the_geom, >st_makeline( >st_pointn(st_exteriorring(the_geom), (rand1.rand * > st_npoints(st_exteriorring(the_geom)))::int), >st_pointn(st_exteriorring(the_geom), (rand2.rand * > st_npoints(st_exteriorring(the_geom)))::int) >) >) >,rand3.rand > ) > from insert_your_table_name_here, > (select random() as rand, generate_series(1,1000) as point_number) as rand1 > JOIN (select random() as rand, generate_series(1,1000) as point_number) as > rand2 >ON rand1.point_number = rand2.point_number > JOIN (select random() as rand, generate_series(1,1000) as point_number) as > rand3 >ON rand2.point_number = rand3.point_number > WHERE st_geometrytype( >st_intersection( >the_geom, >st_makeline( >st_pointn(st_exteriorring(the_geom), (rand1.rand * > st_npoints(st_exteriorring(the_geom)))::int), >st_pointn(st_exteriorring(the_geom), (rand2.rand * > st_npoints(st_exteriorring(the_geom)))::int) >) >) > ) = 'ST_LineString' > AND oid = 5030 /* Enter your own OID here */ > limit 100 > > > > >-Original Message- > >From: postgis-users-boun...@postgis.refractions.net > >[mailto:postgis-users-boun...@postgis.refractions.net] On > >Behalf Of Martin Davis > >Sent: Thursday, May 06, 2010 8:56 AM > >To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis > >Subject: Re: [postgis-users] Dot Density idea > > > > > >I was thinking the same thing! > > > >strk wrote: > >> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting > >> function indeed. Sounds like a good job for GEOS/JTS. > >> > >> --strk; > >> > >> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: > >> > >>> One of the things I miss about using ESRI's GIS is the > >ability to do > >>> dot-density maps. Within a polygon, the number of dots is > >proportional to a value, and the dots are randomly placed. I > >find it useful to be able to present several data values at > >once (e.g. blue dots for population, red dots for employment). > >>> > >>> I also find that it is a more intuitive way of scaling for > >zone size > >>> than dividing the value by the area of the zone. That is, > >the count > >>> of the dots represents the actual number, but the density > >of the dots > >>> represents the density of the number. So I don't have to decide > >>> whether to divide the value by the area of the polygon to plot > >>> density: both the absolute number and the density are > >easily visible. > >>> > >>> Since my open-source GIS viewing systems (mostly QGIS and > >Mapserver) > >>> won't plot dot-density, I've done without. > >>> > >>> But today I realized that I can build these on the server > >instead. I > >>> can generate random points within the bounding-box of the polygon, > >>> throwing out those that aren't contained within the polygon, > >>> repeating until I have enough. Then I can save these points as a > >>> separate layer, and display this layer using almost any desktop or > >>> web based viewer! > >>> > >>> Has anyone done this? Can I do it in SQL or do I need to write > >>> something in PL/pgsql? > >>> > >>> -- > >>> John Abraham > >>> > >>> PS I just bought the Postgis In Action book; enjoying it so far. > >>> ___ > >>> postgis-users mailing list postgis-users@postgis.refractions.net > >>> http://postgis.refractions.net/mailman/listinfo/postgis-users > >>> > >> > >> > > > >-- > >Martin Davis > >Senior Technical Architect > >Refractions Research, Inc. > >(250) 383-3022 > > > >___ > >postgis-users mailing list > >postgis-users@postgis.refractions.net > >http://postgis.refractions.net/mailman/listinfo/postgis-users > > > ___ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > -- George R. C. Silva Desenvolvimento em GIS http://blog.geoprocessamento.net ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
Looks nasty, but it might work: select st_line_interpolate_point( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ,rand3.rand ) from insert_your_table_name_here, (select random() as rand, generate_series(1,1000) as point_number) as rand1 JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand2 ON rand1.point_number = rand2.point_number JOIN (select random() as rand, generate_series(1,1000) as point_number) as rand3 ON rand2.point_number = rand3.point_number WHERE st_geometrytype( st_intersection( the_geom, st_makeline( st_pointn(st_exteriorring(the_geom), (rand1.rand * st_npoints(st_exteriorring(the_geom)))::int), st_pointn(st_exteriorring(the_geom), (rand2.rand * st_npoints(st_exteriorring(the_geom)))::int) ) ) ) = 'ST_LineString' AND oid = 5030 /* Enter your own OID here */ limit 100 >-Original Message- >From: postgis-users-boun...@postgis.refractions.net >[mailto:postgis-users-boun...@postgis.refractions.net] On >Behalf Of Martin Davis >Sent: Thursday, May 06, 2010 8:56 AM >To: John Abraham; postgis-users@postgis.refractions.net; Martin Davis >Subject: Re: [postgis-users] Dot Density idea > > >I was thinking the same thing! > >strk wrote: >> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting >> function indeed. Sounds like a good job for GEOS/JTS. >> >> --strk; >> >> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: >> >>> One of the things I miss about using ESRI's GIS is the >ability to do >>> dot-density maps. Within a polygon, the number of dots is >proportional to a value, and the dots are randomly placed. I >find it useful to be able to present several data values at >once (e.g. blue dots for population, red dots for employment). >>> >>> I also find that it is a more intuitive way of scaling for >zone size >>> than dividing the value by the area of the zone. That is, >the count >>> of the dots represents the actual number, but the density >of the dots >>> represents the density of the number. So I don't have to decide >>> whether to divide the value by the area of the polygon to plot >>> density: both the absolute number and the density are >easily visible. >>> >>> Since my open-source GIS viewing systems (mostly QGIS and >Mapserver) >>> won't plot dot-density, I've done without. >>> >>> But today I realized that I can build these on the server >instead. I >>> can generate random points within the bounding-box of the polygon, >>> throwing out those that aren't contained within the polygon, >>> repeating until I have enough. Then I can save these points as a >>> separate layer, and display this layer using almost any desktop or >>> web based viewer! >>> >>> Has anyone done this? Can I do it in SQL or do I need to write >>> something in PL/pgsql? >>> >>> -- >>> John Abraham >>> >>> PS I just bought the Postgis In Action book; enjoying it so far. >>> ___ >>> postgis-users mailing list postgis-users@postgis.refractions.net >>> http://postgis.refractions.net/mailman/listinfo/postgis-users >>> >> >> > >-- >Martin Davis >Senior Technical Architect >Refractions Research, Inc. >(250) 383-3022 > >___ >postgis-users mailing list >postgis-users@postgis.refractions.net >http://postgis.refractions.net/mailman/listinfo/postgis-users > ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
I was thinking the same thing! strk wrote: ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: One of the things I miss about using ESRI's GIS is the ability to do dot-density maps. Within a polygon, the number of dots is proportional to a value, and the dots are randomly placed. I find it useful to be able to present several data values at once (e.g. blue dots for population, red dots for employment). I also find that it is a more intuitive way of scaling for zone size than dividing the value by the area of the zone. That is, the count of the dots represents the actual number, but the density of the dots represents the density of the number. So I don't have to decide whether to divide the value by the area of the polygon to plot density: both the absolute number and the density are easily visible. Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't plot dot-density, I've done without. But today I realized that I can build these on the server instead. I can generate random points within the bounding-box of the polygon, throwing out those that aren't contained within the polygon, repeating until I have enough. Then I can save these points as a separate layer, and display this layer using almost any desktop or web based viewer! Has anyone done this? Can I do it in SQL or do I need to write something in PL/pgsql? -- John Abraham PS I just bought the Postgis In Action book; enjoying it so far. ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users -- Martin Davis Senior Technical Architect Refractions Research, Inc. (250) 383-3022 ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
I have used an R function before (using maptools package) to do something similar, but I can't remember the details, so I would be considering looking at PL/R. cheers Ben On 06/05/2010, at 15:40 , strk wrote: > On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: >> One of the things I miss about using ESRI's GIS is the ability to do >> dot-density maps. Within a polygon, the number of dots is proportional to a >> value, and the dots are randomly placed. I find it useful to be able to >> present several data values at once (e.g. blue dots for population, red dots >> for employment). >> >> I also find that it is a more intuitive way of scaling for zone size than >> dividing the value by the area of the zone. That is, the count of the dots >> represents the actual number, but the density of the dots represents the >> density of the number. So I don't have to decide whether to divide the >> value by the area of the polygon to plot density: both the absolute number >> and the density are easily visible. >> >> Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't >> plot dot-density, I've done without. >> >> But today I realized that I can build these on the server instead. I can >> generate random points within the bounding-box of the polygon, throwing out >> those that aren't contained within the polygon, repeating until I have >> enough. Then I can save these points as a separate layer, and display this >> layer using almost any desktop or web based viewer! >> >> Has anyone done this? Can I do it in SQL or do I need to write something in >> PL/pgsql? > > PL/pgsql would be easier than SQL at the minimum. > Still, a C implementation would likely be better, for speed reasons. > > --strk; > > () Free GIS & Flash consultant/developer > /\ http://strk.keybit.net/services.html > ___ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
Not quite the same, but Carson Farmer's fTools plugins for QGIS include the functionality to create random points (by count or density) inside of a polygon. http://www.ftools.ca/plugins.html Nothing specific about this functionality on this page, you need to look at the actual plugins in QGIS. David. On Thu, May 6, 2010 at 4:10 AM, strk wrote: > ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting > function indeed. Sounds like a good job for GEOS/JTS. > > --strk; > > On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: >> One of the things I miss about using ESRI's GIS is the ability to do >> dot-density maps. Within a polygon, the number of dots is proportional to a >> value, and the dots are randomly placed. I find it useful to be able to >> present several data values at once (e.g. blue dots for population, red dots >> for employment). >> >> I also find that it is a more intuitive way of scaling for zone size than >> dividing the value by the area of the zone. That is, the count of the dots >> represents the actual number, but the density of the dots represents the >> density of the number. So I don't have to decide whether to divide the >> value by the area of the polygon to plot density: both the absolute number >> and the density are easily visible. >> >> Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't >> plot dot-density, I've done without. >> >> But today I realized that I can build these on the server instead. I can >> generate random points within the bounding-box of the polygon, throwing out >> those that aren't contained within the polygon, repeating until I have >> enough. Then I can save these points as a separate layer, and display this >> layer using almost any desktop or web based viewer! >> >> Has anyone done this? Can I do it in SQL or do I need to write something in >> PL/pgsql? >> >> -- >> John Abraham >> >> PS I just bought the Postgis In Action book; enjoying it so far. >> ___ >> postgis-users mailing list >> postgis-users@postgis.refractions.net >> http://postgis.refractions.net/mailman/listinfo/postgis-users > > -- > > () Free GIS & Flash consultant/developer > /\ http://strk.keybit.net/services.html > ___ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting function indeed. Sounds like a good job for GEOS/JTS. --strk; On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: > One of the things I miss about using ESRI's GIS is the ability to do > dot-density maps. Within a polygon, the number of dots is proportional to a > value, and the dots are randomly placed. I find it useful to be able to > present several data values at once (e.g. blue dots for population, red dots > for employment). > > I also find that it is a more intuitive way of scaling for zone size than > dividing the value by the area of the zone. That is, the count of the dots > represents the actual number, but the density of the dots represents the > density of the number. So I don't have to decide whether to divide the value > by the area of the polygon to plot density: both the absolute number and the > density are easily visible. > > Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't > plot dot-density, I've done without. > > But today I realized that I can build these on the server instead. I can > generate random points within the bounding-box of the polygon, throwing out > those that aren't contained within the polygon, repeating until I have > enough. Then I can save these points as a separate layer, and display this > layer using almost any desktop or web based viewer! > > Has anyone done this? Can I do it in SQL or do I need to write something in > PL/pgsql? > > -- > John Abraham > > PS I just bought the Postgis In Action book; enjoying it so far. > ___ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users -- () Free GIS & Flash consultant/developer /\ http://strk.keybit.net/services.html ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] Dot Density idea
On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote: > One of the things I miss about using ESRI's GIS is the ability to do > dot-density maps. Within a polygon, the number of dots is proportional to a > value, and the dots are randomly placed. I find it useful to be able to > present several data values at once (e.g. blue dots for population, red dots > for employment). > > I also find that it is a more intuitive way of scaling for zone size than > dividing the value by the area of the zone. That is, the count of the dots > represents the actual number, but the density of the dots represents the > density of the number. So I don't have to decide whether to divide the value > by the area of the polygon to plot density: both the absolute number and the > density are easily visible. > > Since my open-source GIS viewing systems (mostly QGIS and Mapserver) won't > plot dot-density, I've done without. > > But today I realized that I can build these on the server instead. I can > generate random points within the bounding-box of the polygon, throwing out > those that aren't contained within the polygon, repeating until I have > enough. Then I can save these points as a separate layer, and display this > layer using almost any desktop or web based viewer! > > Has anyone done this? Can I do it in SQL or do I need to write something in > PL/pgsql? PL/pgsql would be easier than SQL at the minimum. Still, a C implementation would likely be better, for speed reasons. --strk; () Free GIS & Flash consultant/developer /\ http://strk.keybit.net/services.html ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
