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 <m...@squarepegsystems.com> 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 > > _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users