[mapguide-internals] Coordinate System question

[Norm Olsen]

The two systems reported in this e-mail are the same.  I suspect that the flavor of the WKT you got from spatialreference.org was not recognized.  This means that the WKT processor did not have a reference to use when mapping names in the WKT (such as the datum name).  Thus, it used a search mechanism to find a MAP system which was identical to the WKT system.  ETRF89.TM31 was the first system definition it found which was the mathematical equivalent to the provided WKT.  SO that is what MAP used.

The two definitions are identical (mathematically).
The information in spatialreference.org is correct.
You can use either one and get the same numerical results.  I you want to be strictly correct with regard to the EPSG code, you should use the ETRS89.UTM-31N as that is what the NameMapper facility will map the provided EPSG code to.

Norm

-----Original Message-----
From: mapguide-internals-bounces at lists.osgeo.org [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Jason Birch
Sent: Sunday, July 11, 2010 12:46 AM
To: MapGuide Internals Mail List
Subject: [mapguide-internals] Coordinate System question

Hi all,

I'm playing around with the benchmarking data, which is apparently provided
in ETRS89, UTM31N.  .prj files were not included with the shape files, so I
looked up the EPSG code that Jeff was using in his WMS example (25831) at
http://spatialreference.org/ref/epsg/25831/ and got this back in a PRJ file:

PROJCS["ETRS89 / UTM zone
31N",GEOGCS["ETRS89",DATUM["D_ETRS_1989",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",3],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["Meter",1]]

I copied this file for each of the shape files, and moved on.

A bit later, when I was loading the shape files into MapGuide, I plugged the
above WKT into Maestro and clicked on Validate (which calls the MapGuide CS
lookup functions), and it came back with ETRF89.TM31, EPSG:3043, which
caught me by surprise.

So, I put the original EPSG code into Maestro, and after validation it came
back with ETRS89.UTM-31N, and the following WKT:

PROJCS["ETRS89.UTM-31N",GEOGCS["LL-ETRF89",DATUM["ETRF89",SPHEROID["GRS1980",6378137.000,298.25722210],TOWGS84[0.0000,0.0000,0.0000,0.000000,0.000000,0.000000,0.00000000]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["false_easting",500000.000],PARAMETER["false_northing",0.000],PARAMETER["central_meridian",3.00000000000000],PARAMETER["scale_factor",0.9996],PARAMETER["latitude_of_origin",0.000],UNIT["Meter",1.00000000000000]]

Are these equivalent?  Is the data on spatialreference.org correct?  Which
CS should I be using?

Thanks,
Jason
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