Re: [postgis-users] Distances off in the Southern US
I'll try to answer everyone's suggestions:
I didn't know that Yahoo's service was in 4326. That did change things up a
bit. And, yes, I did copy the value from Birmingham to Miami twice. Sorry
about that. . .
My customer was comparing my results to a different source but it seems
close to Google Maps. I know that I was using the road miles in Birmingham
to Miami (
and other southern cites) but my other mileage to the northern cities was
>5% off from the Google Maps was. So that I was kinda close.
Now I combined a lot of the suggestions, so now my Geometry looks like:
select AddGeometryColumn('cities','geom',2163,'POINT',2);
UPDATE cities SET geom=transform(setsrid(makepoint(longitude,
latitude),4326),2163
Birmingham to Miami is:
leader=# select distance( (select geom from cities where id=104), (select
geom from cities where id=41) )*0.000621371192 as miles;
miles
--
656.738138375537
(1 row)
Which is around 13% off in the other direction.
But McHenry to Dallas is now off from what I had:
leader=# select distance( (select geom from cities where id=26251), (select
geom from cities where id=67) )*0.000621371192 as miles;
miles
---
808.949257448
(1 row)
it was 997, which was close to Google Maps. Now it's further off.
But I understand what everyone is saying, and makes sense that flying
distance would be shorter than the road distance. I think I'm comparing
apples to oranges and it was by chance that some random samples I tested
with were close. I think I need to find some interstate shapefiles, put
them in, and have PostGIS calculate the distance between those points on
that route.
Thanks everyone.
On Mon, Aug 15, 2011 at 11:04 AM, Paul Ramsey wrote:
> You're using polar stereograpic as your projection?
>
> http://nsidc.org/data/atlas/epsg_32661.html
>
> Well, yes, the farther south you go, the more over-determined your
> distance will be.
>
> Use the geography type, as suggested below, or the
> st_distance_spheroid() function, or just use a projection that is more
> conformal for the area of interest (continental USA?
> http://prj2epsg.org/epsg/2163)
>
> P.
>
> On Mon, Aug 15, 2011 at 3:50 AM, Mike Hostetler
> wrote:
> > Hello,
> > I'm somewhat new to GIS and I have a problem that I thought appeared to
> be
> > simply using a wrong projection or datum, but it seems to be a bit more
> > subtle than that.
> > I have a table of cities in the US and I'm trying to find distances
> between
> > them. When I use a city that is in the northern US, it works fine. When
> I
> > try to find the distance between two cities in the Southern US, the
> distance
> > becomes way off.
> > I setup a Geometry in my cities table and populated it like the
> following:
> > select AddGeometryColumn('cities','geom',32661,'POINT',2);
> > UPDATE cities SET geom=transform(setsrid(makepoint(longitude,
> > latitude),4269), 32661)
> > (I find the latitude and longitude from the Yahoo Geocode service)
> > A distance calc from McHenry, IL to Dallas, TX is calculated as:
> > select distance( (select geom from cities where id=26251), (select geom
> from
> > cities where id=67) )*0.000621371192 as miles;
> > miles
> > --
> > 996.717850542391
> > (Google Maps reads as 972, off by 25 miles or off around 4%)
> >
> > But Birmingham, AL, to Miami, FL is calculated as:
> > leader=# select distance( (select geom from cities where id=26251),
> (select
> > geom from cities where id=67) )*0.000621371192 as miles;
> > miles
> > --
> > 996.717850542391
> > (Google Maps reads as 767, off by 120 files, or 13%).
> > I can handle a little error, as long as it's somewhat small (<5%). But
> this
> > is way off.
> > Again, it smells to be to be a datum or projection issue to me, but I'm
> not
> > sure how to find the sweet spot to be accurate everywhere.
> > Your input is appreciated.
> >
> > --
> > Mike Hostetler
> > SquarePeg Systems
> > http://www.squarepegsystems.com
> >
> > ___
> > postgis-users mailing list
> > postgis-users@postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> >
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--
Mike Hostetler
SquarePeg Systems
http://www.squarepegsystems.com
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Re: [postgis-users] Distances off in the Southern US
You're using polar stereograpic as your projection?
http://nsidc.org/data/atlas/epsg_32661.html
Well, yes, the farther south you go, the more over-determined your
distance will be.
Use the geography type, as suggested below, or the
st_distance_spheroid() function, or just use a projection that is more
conformal for the area of interest (continental USA?
http://prj2epsg.org/epsg/2163)
P.
On Mon, Aug 15, 2011 at 3:50 AM, Mike Hostetler
wrote:
> Hello,
> I'm somewhat new to GIS and I have a problem that I thought appeared to be
> simply using a wrong projection or datum, but it seems to be a bit more
> subtle than that.
> I have a table of cities in the US and I'm trying to find distances between
> them. When I use a city that is in the northern US, it works fine. When I
> try to find the distance between two cities in the Southern US, the distance
> becomes way off.
> I setup a Geometry in my cities table and populated it like the following:
> select AddGeometryColumn('cities','geom',32661,'POINT',2);
> UPDATE cities SET geom=transform(setsrid(makepoint(longitude,
> latitude),4269), 32661)
> (I find the latitude and longitude from the Yahoo Geocode service)
> A distance calc from McHenry, IL to Dallas, TX is calculated as:
> select distance( (select geom from cities where id=26251), (select geom from
> cities where id=67) )*0.000621371192 as miles;
> miles
> --
> 996.717850542391
> (Google Maps reads as 972, off by 25 miles or off around 4%)
>
> But Birmingham, AL, to Miami, FL is calculated as:
> leader=# select distance( (select geom from cities where id=26251), (select
> geom from cities where id=67) )*0.000621371192 as miles;
> miles
> --
> 996.717850542391
> (Google Maps reads as 767, off by 120 files, or 13%).
> I can handle a little error, as long as it's somewhat small (<5%). But this
> is way off.
> Again, it smells to be to be a datum or projection issue to me, but I'm not
> sure how to find the sweet spot to be accurate everywhere.
> Your input is appreciated.
>
> --
> Mike Hostetler
> SquarePeg Systems
> http://www.squarepegsystems.com
>
> ___
> postgis-users mailing list
> postgis-users@postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>
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Re: [postgis-users] Distances off in the Southern US
Hi Mike and Nicolas,
The fact that Mike's two calculations resulted in the same value to 12
decimal places is more that a little fishy. I seem to remember a similar
issue coming up on this list sometime ago (roughly a year ago is my,
faulty, memory). The difference between southern and northern cities in
the US just shouldn't be an issue if the data was read in correctly.
Dan
On 08/15/2011 06:18 AM, Nicolas Ribot wrote:
On 15 August 2011 12:50, Mike Hostetler wrote:
Hello,
I'm somewhat new to GIS and I have a problem that I thought appeared to be
simply using a wrong projection or datum, but it seems to be a bit more
subtle than that.
I have a table of cities in the US and I'm trying to find distances between
them. When I use a city that is in the northern US, it works fine. When I
try to find the distance between two cities in the Southern US, the distance
becomes way off.
I setup a Geometry in my cities table and populated it like the following:
select AddGeometryColumn('cities','geom',32661,'POINT',2);
UPDATE cities SET geom=transform(setsrid(makepoint(longitude,
latitude),4269), 32661)
(I find the latitude and longitude from the Yahoo Geocode service)
A distance calc from McHenry, IL to Dallas, TX is calculated as:
select distance( (select geom from cities where id=26251), (select geom from
cities where id=67) )*0.000621371192 as miles;
miles
--
996.717850542391
(Google Maps reads as 972, off by 25 miles or off around 4%)
But Birmingham, AL, to Miami, FL is calculated as:
leader=# select distance( (select geom from cities where id=26251), (select
geom from cities where id=67) )*0.000621371192 as miles;
miles
--
996.717850542391
(Google Maps reads as 767, off by 120 files, or 13%).
I can handle a little error, as long as it's somewhat small (<5%). But this
is way off.
Again, it smells to be to be a datum or projection issue to me, but I'm not
sure how to find the sweet spot to be accurate everywhere.
Your input is appreciated.
Hi Mike,
Some remarks:
• Are you sure Yahoo! Geocoder Service returns degree expressed on
4269 coordinate system ? I read it is 4326.
• How did you get the distances with Google Maps ? Did you use travel
directions ? If so, the 972 miles is by driving on roads, not by
flying the shortest distance (great circle).
The following service:
http://www.geobytes.com/CityDistanceTool.htm?loadpage
gave me a direct distance of 808 miles or 1300km for McHenry - Dallas
and a distance of 658 miles or 1059 km for Birmingham - Miami.
Then, by running these queries the distance between cities seems to be good:
select foo.city, bar.city, st_distance(foo.geom,
bar.geom)*0.000621371192 as miles
from
(select 'McHenry' as city, 'IL' as state, 'POINT(-88.267314
42.326215)'::geography as geom) as foo,
(select 'Dallas' as city, 'TX' as state, 'POINT(-96.795404
32.778155)'::geography as geom) as bar;
city | city | miles
-++--
McHenry | Dallas | 807.029700174124
select foo.city, bar.city, st_distance(foo.geom,
bar.geom)*0.000621371192 as miles
from
(select 'Birmingham' as city, 'AL' as state, 'POINT(-86.811504
33.520295)'::geography as geom) as foo,
(select 'Miami' as city, 'FL' as state, 'POINT(-80.237419
25.728985)'::geography as geom) as bar
city| city | miles
+---+--
Birmingham | Miami | 666.318599210394
The slight differences between CityDistanceTool and Yahoo! services
come from the cities coordinates: the two services do not position
cities at the same coordinates.
Also please note I'm using the new GEOGRAPHY Postgis type that allows
direct distance computation: no need to transform data back to a
planar coordinate system.
If you use geometry type with long/lat coordinates, you may have a
look at st_distanceSphere and st_distanceSpheroid.
Nicolas
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Re: [postgis-users] Distances off in the Southern US
On 15 August 2011 12:50, Mike Hostetler wrote:
> Hello,
> I'm somewhat new to GIS and I have a problem that I thought appeared to be
> simply using a wrong projection or datum, but it seems to be a bit more
> subtle than that.
> I have a table of cities in the US and I'm trying to find distances between
> them. When I use a city that is in the northern US, it works fine. When I
> try to find the distance between two cities in the Southern US, the distance
> becomes way off.
> I setup a Geometry in my cities table and populated it like the following:
> select AddGeometryColumn('cities','geom',32661,'POINT',2);
> UPDATE cities SET geom=transform(setsrid(makepoint(longitude,
> latitude),4269), 32661)
> (I find the latitude and longitude from the Yahoo Geocode service)
> A distance calc from McHenry, IL to Dallas, TX is calculated as:
> select distance( (select geom from cities where id=26251), (select geom from
> cities where id=67) )*0.000621371192 as miles;
> miles
> --
> 996.717850542391
> (Google Maps reads as 972, off by 25 miles or off around 4%)
>
> But Birmingham, AL, to Miami, FL is calculated as:
> leader=# select distance( (select geom from cities where id=26251), (select
> geom from cities where id=67) )*0.000621371192 as miles;
> miles
> --
> 996.717850542391
> (Google Maps reads as 767, off by 120 files, or 13%).
> I can handle a little error, as long as it's somewhat small (<5%). But this
> is way off.
> Again, it smells to be to be a datum or projection issue to me, but I'm not
> sure how to find the sweet spot to be accurate everywhere.
> Your input is appreciated.
>
Hi Mike,
Some remarks:
• Are you sure Yahoo! Geocoder Service returns degree expressed on
4269 coordinate system ? I read it is 4326.
• How did you get the distances with Google Maps ? Did you use travel
directions ? If so, the 972 miles is by driving on roads, not by
flying the shortest distance (great circle).
The following service:
http://www.geobytes.com/CityDistanceTool.htm?loadpage
gave me a direct distance of 808 miles or 1300km for McHenry - Dallas
and a distance of 658 miles or 1059 km for Birmingham - Miami.
Then, by running these queries the distance between cities seems to be good:
select foo.city, bar.city, st_distance(foo.geom,
bar.geom)*0.000621371192 as miles
from
(select 'McHenry' as city, 'IL' as state, 'POINT(-88.267314
42.326215)'::geography as geom) as foo,
(select 'Dallas' as city, 'TX' as state, 'POINT(-96.795404
32.778155)'::geography as geom) as bar;
city | city | miles
-++--
McHenry | Dallas | 807.029700174124
select foo.city, bar.city, st_distance(foo.geom,
bar.geom)*0.000621371192 as miles
from
(select 'Birmingham' as city, 'AL' as state, 'POINT(-86.811504
33.520295)'::geography as geom) as foo,
(select 'Miami' as city, 'FL' as state, 'POINT(-80.237419
25.728985)'::geography as geom) as bar
city| city | miles
+---+--
Birmingham | Miami | 666.318599210394
The slight differences between CityDistanceTool and Yahoo! services
come from the cities coordinates: the two services do not position
cities at the same coordinates.
Also please note I'm using the new GEOGRAPHY Postgis type that allows
direct distance computation: no need to transform data back to a
planar coordinate system.
If you use geometry type with long/lat coordinates, you may have a
look at st_distanceSphere and st_distanceSpheroid.
Nicolas
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[postgis-users] Distances off in the Southern US
Hello,
I'm somewhat new to GIS and I have a problem that I thought appeared to be
simply using a wrong projection or datum, but it seems to be a bit more
subtle than that.
I have a table of cities in the US and I'm trying to find distances between
them. When I use a city that is in the northern US, it works fine. When I
try to find the distance between two cities in the Southern US, the distance
becomes way off.
I setup a Geometry in my cities table and populated it like the following:
select AddGeometryColumn('cities','geom',32661,'POINT',2);
UPDATE cities SET geom=transform(setsrid(makepoint(longitude,
latitude),4269), 32661)
(I find the latitude and longitude from the Yahoo Geocode service)
A distance calc from McHenry, IL to Dallas, TX is calculated as:
select distance( (select geom from cities where id=26251), (select geom from
cities where id=67) )*0.000621371192 as miles;
miles
--
996.717850542391
(Google Maps reads as 972, off by 25 miles or off around 4%)
But Birmingham, AL, to Miami, FL is calculated as:
leader=# select distance( (select geom from cities where id=26251), (select
geom from cities where id=67) )*0.000621371192 as miles;
miles
--
996.717850542391
(Google Maps reads as 767, off by 120 files, or 13%).
I can handle a little error, as long as it's somewhat small (<5%). But this
is way off.
Again, it smells to be to be a datum or projection issue to me, but I'm not
sure how to find the sweet spot to be accurate everywhere.
Your input is appreciated.
--
Mike Hostetler
SquarePeg Systems
http://www.squarepegsystems.com
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