[FWTools] Comparing SRS objects

[Ed McNierney]

David -

In my (limited) experience I have found this an exceptionally difficult
thing to do.  I found it hard enough that I gave up!

We used to offer an ArcIMS interface to our WMS image services.  We
created an ArcIMS emulator that, in essence, translated ArcIMS image
requests to WMS image requests.  This worked fine until ESRI changed the
ArcGIS client interface so it specified the image SRS in WKT format
rather than with an EPSG index.  Since the WMS interface required an
EPSG index, we needed to translate arbitrary WKT descriptions (esp.
ESRI-flavored ones) into EPSG codes.  Oy!  Since there is a reasonable
degree of flexibility in a WKT description, it is quite easy to come up
with two WKT projections that are, in practice, "the same projection"
but which look different.

Fortunately, that was the same release in which ESRI added real OGC WMS
client support to ArcGIS, so we simply surrendered and discontinued
ArcIMS support, and provided support exclusively for our OGC WMS
interface.

This is one of the chief benefits, IMHO, of the EPSG or any other
catalog-style system.  EPSG:4326 is the same as EPSG:4326 and is not the
same as EPSG:26915 and that's pretty darn easy to figure out.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: fwtools-bounces at lists.maptools.org
[mailto:fwtools-bounces at lists.maptools.org] On Behalf Of Frank Warmerdam
Sent: Monday, November 20, 2006 4:26 PM
To: David Fawcett
Cc: fwtools at lists.maptools.org
Subject: Re: [FWTools] Comparing SRS objects

David Fawcett wrote:
> In a Python script, I want to compare spatial reference systems.  For 
> context, I am creating a map image with Python MapScript, the map SRS 
> is UTM, the layer SRS for one of the layers could sometimes be in UTM 
> or sometimes in Lat/Long.  The extent of the map will be based on this
layer.
> 
> As I build the layer, I want to check to see what SRS it is in, if it 
> is in Lat/Long, I am planning to grab it's extent in lat/long, create 
> a rectObj, and then project this rectObj to UTM so I can use the 
> coordinates for my map extent.
> 
> Assuming that this is a reasonable way to approach the problem, I am 
> curious how to use OGR, OSR, or MapScript to compare SRS's.  If I 
> create SRS objects, can I just check that they are equal, as below, or

> do I need to dump them to WKT or proj Defs and do a string comparison?
> 
> e.g.
> 
>     srs_one = osr.SpatialReference()
>     srs_two = osr.SpatialReference()
> 
>     srs_one .ImportFromEPSG(4326)
>     srs_two .ImportFromEPSG(26915)
> 
>     if srs_one == srs_two:
>         print "they are the same"
>     else:
>         print "they are different"

David,

I think this will only determine if they are the same object, not if the
coordinate system description is the same.  There is an IsSame() method
on the osr.SpatialReference, so you could instead do:
       if srs_one.IsSame(srs_two):
          ...

that said, the IsSame() method is not really very robust.

Another approach would be to convert them both to WKT and compare those
strings, though you will get a lot of apparent differences which are not
actually significant to the underlying definition.

Best regards,
-- 
---------------------------------------+--------------------------------
---------------------------------------+------
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo,
http://osgeo.org

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