[Geotiff] Geocentric Coordinates and GeoTIFF

[Frank Warmerdam]

Folks,

The LAS Lidar format uses geotiff tags to represent the coordinate
system and in that format is sometimes useful to be able to represent
a geocentric (ECEF) coordinate system.  Reviewing the GeoTIFF specification
it seems there was some attempt to address this including a
ModelTypeGeocentric.  So I have gone ahead and tried representing a
geocentric coordinate system in GeoTIFF tags.  The result I get is:

Geotiff_Information:
    Version: 1
    Key_Revision: 1.0
    Tagged_Information:
       End_Of_Tags.
    Keyed_Information:
       GTModelTypeGeoKey (Short,1): ModelTypeGeocentric
       GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
       GTCitationGeoKey (Ascii,20): "WGS 84 (geocentric)"
       GeographicTypeGeoKey (Short,1): GCS_WGS_84
       GeogLinearUnitsGeoKey (Short,1): Linear_Meter
       GeogAngularUnitsGeoKey (Short,1): Angular_Degree
       GeogSemiMajorAxisGeoKey (Double,1): 6378137
       GeogInvFlatteningGeoKey (Double,1): 298.257223563
       End_Of_Keys.
    End_Of_Geotiff.

This corresponds to the WKT:

GEOCCS["WGS 84 (geocentric)",
     DATUM["WGS_1984",
         SPHEROID["WGS 84",6378137,298.257223563,
             AUTHORITY["EPSG","7030"]],
         AUTHORITY["EPSG","6326"]],
     PRIMEM["Greenwich",0,
         AUTHORITY["EPSG","8901"]],
     UNIT["metre",1,
         AUTHORITY["EPSG","9001"]],
     AXIS["Geocentric X",OTHER],
     AXIS["Geocentric Y",OTHER],
     AXIS["Geocentric Z",NORTH],
     AUTHORITY["EPSG","4328"]]

Interestingly, I was not able to determine an appropriate GeoTIFF tag
to use to represent the overall Geocentric coordinate system.  In this
case the coordinate system is EPSG:4328.  But it does not appear to be
appropriate to place this value in either the GeographicTypeGeoKey or
the ProjectedCSTypeGeoKey.  Has anyone else tried to work with
Geocentric coordinate systems in GeoTIFF?  Any examples of good practice
to compare?

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 Programmer for Rent