[Geotiff] Mercator Clarification

[Frank Warmerdam]

Folks,

I was recently provided with a GeoTIFF file with this set of keys:

Geotiff_Information:
    Version: 1
    Key_Revision: 1.0
    Tagged_Information:
       ModelTiepointTag (2,3):
          874              434              0
          -1237.52006      4190457.16       0
       ModelPixelScaleTag (1,3):
          -1237.52006      1237.52006       0
       End_Of_Tags.
    Keyed_Information:
       GTModelTypeGeoKey (Short,1): ModelTypeProjected
       GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
       GeogGeodeticDatumGeoKey (Short,1): User-Defined
       GeogPrimeMeridianGeoKey (Short,1): PM_Greenwich
       GeogAngularUnitsGeoKey (Short,1): Angular_Degree
       GeogEllipsoidGeoKey (Short,1): User-Defined
       GeogSemiMajorAxisGeoKey (Double,1): 6378137
       GeogSemiMinorAxisGeoKey (Double,1): 6356752.31
       ProjectedCSTypeGeoKey (Short,1): User-Defined
       PCSCitationGeoKey (Ascii,14): "METOC-Halifax"
       ProjectionGeoKey (Short,1): User-Defined
       ProjCoordTransGeoKey (Short,1): CT_Mercator
       ProjLinearUnitsGeoKey (Short,1): Linear_Meter
       ProjNatOriginLongGeoKey (Double,1): -59.5
       ProjNatOriginLatGeoKey (Double,1): 39
       ProjFalseEastingGeoKey (Double,1): 0
       ProjFalseNorthingGeoKey (Double,1): 0
       End_Of_Keys.
    End_Of_Geotiff.

It seems it is intended to be a Mercator 2SP projection with 39 as
the standard parallel (or latitude of true scale).  It seems to me that
encoding the standard parallel as NatOriginLat is an error.

The mercator pages I have been maintaining seem quite mixed up on this
topic:

   http://geotiff.maptools.org/proj_list/mercator_1sp.html
   http://geotiff.maptools.org/proj_list/mercator_2sp.html

I'm seeking to clarify that Mercator 1SP would have the GeoTIFF
parameters:

   CT_Mercator
   NatOriginLong (aka Central Meridian)
   ScaleAtNatOrigin
   FalseEasting
   FalseNorthing

and that Mercator 2SP would have the geotiff parameters:

   CT_Mercator
   NatOriginLong
   StdParallel1  (ie. latitude of true scale)
   FalseEasting
   FalseNorthing

Essentially the forms are equivelent as the 2SP version can be turned
into the 1SP version by computing the equivelent scale at the equator
that corresponds with having the described standard parallel being the
latitude of true scale.

Matters have been muddied (in my mind at least) by the NatOriginLat
parameter.  Is NatOriginLat intended to be the same as StdParalle1?
Is it just another way of establishing what latitude is "zero northing"
(aka an alternate formulation of FalseNorthing)? Does it have some
other meaning?

There does not seem to be any apparent mapping for NatOriginLat in
PROJ.4 or GCTP, so for my purposes if it is a distinct parameter I
will just have to discard it since I have no way of using it.

Input welcome.

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