[postgis-users] Convert between SRID units and meters

[Peter Hsu]

Dan,

Thanks, that was a really good intro to get my head wrapped around some of the concepts, as this is all new to me.

Instead of reprojecting my data, I found that I could use the ST_Transform and ::geography cast to have ST_Area return the geography area instead, a la (snippet follows).  This is fine, and performance isn't an issue as I'm working with this data statically.  

select ST_Area(ST_Transform(the_geom,4326)::geography) AS area from ... <snip>

Hope this helps someone else.. 

Thanks again!

On May 20, 2011, at 12:00 PM, postgis-users-request at postgis.refractions.net wrote:
Hi Peter,

Here is a link to OSGeo workshop that should be a helpful premier on the 
issue in general and provides a solution to your problem through the use 
of PostGIS's geography type: 
http://workshops.opengeo.org/postgis-intro/geography.html

If you are dealing with a fairly well define area, say the West Coast of 
the US and Canada, then you might consider reprojecting your data out of 
NAD83 (North American Datum of 1983) geographic (lon/lat) coordinates 
(EPSG: 4269) into Euclidean coordinates (such as NAD83 UTM 10N), and 
then you can use the standard st_area tool and the result will be in meters.

Dan

On 05/20/2011 11:57 AM, Peter Hsu wrote:
> I have a simple question.  I've loaded up shape files from the tigerline US census bureau.  Everything works fine, but I'm trying to find the area of a geometry in meters.  However, I'm only able to get the area in SRID units.
>
> How can I convert from SRID units to meters?  Is this a linear conversion?  Or is it dependent on the latitude of the area in question?
>
> The SRID for the tigerline files is 4269
>
> Peter
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