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 <mbda...@refractions.net> >>> 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. >>>>>>>>> _______________________________________________ >>>>>>>>> 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 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 >>> >>> >> > > -- > 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
