[gdal-dev] Hotine Oblique Mercator (EPSG:9812) vs. Oblique Mercator (EPSG:9815)

[Frank Warmerdam]

Folks,

There has been a great deal of confusion in my mind about the
two EPSG variants of oblique mercator for over a decade.  Lots
of folks (Roger Lott, and others) have tried to explain this to me
but I basically punted on it.

Mikael Rittri has recently very patiently explained it to me
several times and simplified it enough that I have finally
addressed it.

In the past I treated EPSG projection methods 9815
and 9812 as the same thing, but in fact in PROJ.4 they
need different flags - 9812 requires the +no_uoff flag.

I have modified OGRSpatialReference to now differentiate
these as "Hotine_Oblique_Mercator" and "Oblique_Mercator".
Previously they were all known as "Hotine_Oblique_Mercator".
I'm not sure how many EPSG coordinate systems are of each
variant, but there seems to be well over half a dozen of each
and some of them are quite important.

The changes to GDAL are addressed in:

  http://trac.osgeo.org/gdal/ticket/2745

The primary changeset is:

  http://trac.osgeo.org/gdal/changeset/24091

There is also a helpful discussion of the issue in the PROJ.4 Trac:

  http://trac.osgeo.org/proj/ticket/104

Outstanding issues include properly handling of the Oblique_Mercator
variant in the GeoTIFF code, and the ESRI translation code.

I do *not* plan to back port these changes to 1.9 as they are quite
disruptive.

Best regards,
-- 
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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    | Geospatial Software Developer