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, <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> > > Date: Thu, 21 Feb 2008 09:56:15 > To:postgis-users@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 > your own, and define your projections in PROJ format. Then add them to the > 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@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > > -- ************************************ David William Bitner
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