[Geotiff] Conversion to Proj for USGS

[Ben Supnik]

Hi Y'all,

In testing the latest version of our app, one of our testers grabbed a 
NAIP GeoJP2K from the USGS, and found that it won't unproject.  (This 
isn't off topic, really! :-)

The spacial data for the GeoJP2K comes from a stub GeoTIFF file embedded 
inside, with an unused 1x1 pixel and GeoTIFF tags that set up the 
coordinate systems, etc.

Apparently the USGS is using "WGS_1984_Web_Mercator_Auxiliary_Sphere", 
which is apparently a cheap spherical mercator projector designed to 
match Google/Bing and other wide-scale commercial mapping services.

I looked at the code, and the problem appears to be that while there is 
a proj string that will unproject this, libgeotiff doesn't know how to 
extract it from the given geo tags.  I'm not sure I blame it - I see 
text-based identification of the projection and an ESRI PE string, but 
no EPSG tag or enumerated description of the projection.

My stupid questions:

- Is there a way to mechanically build the projection string from 
anything in this set of GeoTIFF tags?
- If not, does anyone have any suggestions?  Our user base are not core 
GIS users, so having them hand-specify spatial parameters or edit the 
spatial information isn't really on the table.

The listgeo dump is listed below.

Thanks!
Ben

Geotiff_Information:
    Version: 1
    Key_Revision: 1.0
    Tagged_Information:
       ModelTiepointTag (2,3):
          0                 0                 0
          -12481700.1911241 4300624.21315169  0
       ModelPixelScaleTag (1,3):
          1                 1                 0
       End_Of_Tags.
    Keyed_Information:
       GTModelTypeGeoKey (Short,1): User-Defined
       GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
       GTCitationGeoKey (Ascii,50): "PCS Name = 
WGS_1984_Web_Mercator_Auxiliary_Sphere"
       GeographicTypeGeoKey (Short,1): GCS_WGS_84
       GeogCitationGeoKey (Ascii,13): "GCS_WGS_1984"
       GeogAngularUnitsGeoKey (Short,1): Angular_Degree
       GeogSemiMajorAxisGeoKey (Double,1): 6378137
       GeogInvFlatteningGeoKey (Double,1): 298.257223563
       PCSCitationGeoKey (Ascii,445): "ESRI PE String = 
PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.017453292519943295]],PROJECTION["Mercator_Auxiliary_Sphere"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0],PARAMETER["Standard_Parallel_1",0.0],PARAMETER["Auxiliary_Sphere_Type",0.0],UNIT["Meter",1.0]]"
       ProjLinearUnitsGeoKey (Short,1): Linear_Meter
       End_Of_Keys.
    End_Of_Geotiff.

GCS: 4326/WGS 84
Datum: 6326/World Geodetic System 1984
Ellipsoid: 7030/WGS 84 (6378137.00,6356752.31)
Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
Projection Linear Units: 9001/metre (1.000000m)

Corner Coordinates:
Upper Left    (-12481700.191, 4300624.213)
Lower Left    (-12481700.191, 4300623.213)
Upper Right   (-12481699.191, 4300624.213)
Lower Right   (-12481699.191, 4300623.213)
Center        (-12481699.691, 4300623.713)

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