[gdal-dev] Add Mercator_variant_A method?

Mon Apr 13 07:01:28 PDT 2015

> I am not sure I follow what this has to do with the projection name in WKT. The newer WKT specification (ISO 19162) relies solely on the identifier to determine which operation method to use to project coordinates, but the older WKT tends to follow OGC 01-009 which lists explicit names to use for the projections. So regardless of what the EPSG registry uses for a name, this is the name that is supposed to be used for the OGC WKT as far as I am concerned:
> OGC 01-009 (http://www.opengeospatial.org/standards/ct <http://www.opengeospatial.org/standards/ct>): "Mercator_1SP"
> 
> Does someone know of another WKT spec that uses that "Mercator variant A" name, or I am missing something?

When we were defining the GeoPackage spec I had hoped to be able to point people to one definitive list of projection names and parameters so we could say ‘use this, end of story’. I remember discussing this with the CRS WKT people at one of the OGC TCs. IIRC they told me there never was a specification that standardised the projection names. The best practice is to use names that are used in some registry, the EPSG one being the most commonly used one.

The coordinate transformation service spec isn’t actually relevant for GeoPackage and is certainly not some normative reference for WKT projection names. I was under the same impression until I again discussed this with the relevant OGC members and was told that it is not.
The spec that is relevant for GeoPackage is the simple features common architecture one (OGC 06-103r4), section 9. That spec lists a number of projection names in annex B, but that annex is informative and non-exhaustive.

So in short there’s, unfortunately, no such thing as ‘standardised’ WKT wrt the projection and parameter names.

> I'm not opposed to mapping that "Mercator variant A" name to SRS_PT_MERCATOR_1SP on WKT import, but I think there's a limit to how far one should go to support non-standard WKT. I am not sure what modern EPSG names have to do with the explicitly named WKT projection strings that are defined in the actual specification, even spatialreference.org <http://spatialreference.org/> is still using "Mercator_1SP" for the OGC WKT.

IIRC spatialreference.org isn’t being actively maintained anymore. If http://trac.osgeo.org/metacrs/browser/sr.org <http://trac.osgeo.org/metacrs/browser/sr.org> is still the source repo, last relevant update is from 5 years ago.
Last time I reviewed the source code, the WKT it displays is produced by proj.4 and the data for it is sourced from proj.4’s extract of the EPSG database so of course that’s going to roundtrip well. :)

Since all this data is being source from the EPSG database anyway it seems best to use that as authoritative source. The main name for EPSG:9804 <http://www.epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:method:EPSG::9804&reportDetail=long&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=> is 'Mercator (variant A)' now. The 'Mercator (1SP)’ name is still present in the database as an alias. The name change dates back to 2010 stating:

> Code: EPSG::2010.058 
> Reporter: Jay Hollingsworth; Schlumberger 
> Request: Review Mercator and Oblique Mercator method names 
> Actions Taken: For methods 9804-05, 9812-13 and 9815, amended name and added alias. For method 9815 also corrected forward formula for az=90deg case. Added method 1044. For projections 12150, 15001, 15031, 19871-72, 19894-95 and 19956-58, in remarks updated method names. For CRS 3375, in remarks inserted missing and removed spurious space.

Best regards,

Pepijn
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