[postgis-users] trouble getting correct units from "area(transform"

[charles.blankenship at gmail.com]

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 <lr at pcorp.us> 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: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
> Charles
> Blankenship
> Sent: Monday, June 01, 2009 8:04 PM
> To: postgis-users at postgis.refractions.net
> 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
>
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