Thank you so much. The areas do not match the qGIS derived area exactly but that appears to be a well known issue. They're certainly close enough for my needs.
The bonus is that I can now tell my boss that open source software is not the problem! I can't thank you enough! Charles On Mon, Jun 1, 2009 at 5:22 PM, Paragon Corporation <[email protected]> wrote: > 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 >
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