Charles, Which SRID did you use to import your data? Sounds like that is the problem. If you are using QGIS to view the data, I am guessing you used the shape file? And it was reading the prj file to determine the native projection, but you overwrote in SPIT 3310 (which is wrong).
For census data -- you should set SRID to 4269 in SPIT (NAD 83 long lat) and then your area transform below should work fine. So bring the data in as SRID=4269 Then transform 3310 as you are doing (national atlas equal area - 2163 might also be suitable and that covers all of US, but probably not as accurate as California Albers) Leo -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Charles Blankenship Sent: Monday, June 01, 2009 8:04 PM To: [email protected] Subject: [postgis-users] trouble getting correct units from "area(transform" Disclaimer: I'm very new to GIS. I want to calculate the areas of school districts in Nevada (and California at some point) using census shape files. http://www.census.gov/geo/cob/bdy/sn/sn00shp/sn32_d00_shp.zip The data is unprojected so I know I must project it onto something good for equal areas. I'm trying NAD83 California Albers (SRID = 3310). It seems to me from reading the documentation and numerous other posts/tutorials, that the following code should return the area of the districts in meters (or something I can convert to meters). select area(transform(the_geom, 3310)) from mytable This returns a column of areas which match my unprojected areas exactly! I've fiddled with setting the constraint to SRID=3310, importing through the qGIS SPIT plugin, selecting into a new table, delete and resetting constraints, and on and on... FWIW, I can load the file into qGIS, set the projection to 3310, and the identify area feature calculates the "derived area" exactly as it should be. I can't for the life of me figure out how to even get into the approximately correct units. (I've tried reprojecting to quite different SRIDs and can see the area change slightly, but it still appears to be in native map units or degrees). Thanks for any assistance, Charles _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
