# [postgis-users] Projection for Calculating Area

David William Bitner david.bitner at gmail.com
Mon Mar 3 20:08:56 PST 2008

```The function UTMZONE() at
http://postgis.refractions.net/support/wiki/index.php?plpgsqlfunctions will
find the UTM zone for you for a given geometry.

On Thu, Feb 28, 2008 at 11:29 PM, <mchapman at texelinc.com> wrote:

> Also, I think utm is 60 zones each 6 degrees of longitude at the equator
> starting at the dateline and progressing easterly around the globe. Further,
> each zone is divided into a northern and southern hemishere.  Then each zone
> is considered a coordinate system in itself where the width of the zone is
> something like 500000 and the height of the coordinate system is 10000000,
> as compared to an angular projection where the bounds are -180, -90, 180,
> 90.
>
> You could create a table of utm zone geometries and do an intersect each
> time you wanted to figure out what zone you needed.
>
> Just some thoughts.  Hope they help.
> Please correct me if I have a flaw in my suggestions.
>
> Martin
> Sent via BlackBerry by AT&T
>
> -----Original Message-----
> From: Dylan Beaudette <dylan.beaudette at gmail.com>
>
> Date: Thu, 21 Feb 2008 09:56:15
> To:postgis-users at postgis.refractions.net
> Subject: Re: [postgis-users] Projection for Calculating Area
>
>
> On Thursday 21 February 2008 09:33:17 am Travis Kirstine wrote:
> > All,
> > We have been developing a web mapping application using google maps as
> > the front end interface with a postgis backend.  Using this
> > application the user can draw a bounding box in google maps and the
> > coordinates are then passed to postgis.  With postgis we then perform
> > a query on the polygons stored in the postgis database to return the
> > area using  ST_Intersection, ST_Transform (usually to UTM) and finally
> > ST_Area.  The application works great but the problem is  that since
> > we are trying to work on a global scale it is difficult to determine
> > what projection (EPSG) to transform the data to calculate an accurate
> > area since our data is in EPSG:4326 and returns decimal degrees.  We
> > can easily transform the query results to a UTM or a Albers projection
> > however the results can be inaccurate since the user may select
> > multiple zones or hemispheres when view the map at a global scale.  Is
> > there a accurate global projection for calculating area?
>
> No. Except maybe a global sinusoidal projection... but-
>
> This touches on one of the fundamental issues with geographic data: when
> you
> want to compute cartesian statistics (length, area, etc. in a planar
> sense)
> you are going to have to make a compromise in
>
> shape
> direction
> area
>
> AND the larger the region you want to work with the more drastic the
> distortion on 2/3 of the above parameters you will encounter.
>
> This is why 'local' projections like UTM are highly valued-- by working
> with a
> very small chunk of the earth you can minimize distortion considerably.
>
> As for a solution: check the mailing list archives for a custom function
> that
> can automatically determine UTM zone based on a lon/lat coordinate pair.
>
> If you have really big shapes that are spanning multiple UTM zones, then
> setting up some kind of pre-processing logic which determines 1 of several
> possible albers equal area conic projections might work. For example, if
> the
> coordinates are near the continental US, use one set of standard parallels
> /
> central lon/lat to setup the projection. Adjust these parameters
> accordingly
> for different (continents?) regions of the world. You should be able to
> either find some existing EPSG codes which are close enough, or just make
> up
> spatial_ref_sys table.
>
> For some thoughts on equal area projections / projection parameters see:
>
> Snyder, J. P. Map Projections- A working Manual U.S. Geologic Survey, U.S.
> Geologic Survey, 1987
>
> Cheers,
>
> Dylan
>
> --
> Dylan Beaudette
> Soil Resource Laboratory
> http://casoilresource.lawr.ucdavis.edu/
> University of California at Davis
> 530.754.7341
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>

--
************************************
David William Bitner
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20080303/9ba1dfbf/attachment.html>
```