[Geotiff] User_Defined Datums

[Frank Warmerdam]

Kieron Flynn wrote:
> Hi All,
>  
> Hopefully somebody out there can help me.
> I need to set up a User Defined Datum. I'm currently trawling my way 
> through the GeoTiff specification document but I have a really tight 
> deadline on this and am moving too slowely.
> So has anybody got a tiff dump of one of these?. I basically just need 
> to know what tags I need to setup, I have all the information like 
> Inverse flattening and Semi Major Access etc
>  
> Many thanks
> Kieron

Kieron,

This is an example of a GeoTIFF definition with stuff pretty much expanded out
includig a detailed definition of the ellipsoid.  Note that a user defined
GCS can have a user defined prime meridian, ellipsoid, and angular units but
you cannot currently provide any sort of datum shift information in GeoTIFF.
The only way this can be related (in directly) is by referring to an EPSG
datum with known shifts from other sources.  I consider this a flaw in GeoTIFF
worth rectifying at some point.

Geotiff_Information:
    Version: 1
    Key_Revision: 1.0
    Tagged_Information:
       ModelTiepointTag (2,3):
          0                0                0
          825006.615       1854996.41       0
       ModelPixelScaleTag (1,3):
          13.2291667       13.2291667       0
       End_Of_Tags.
    Keyed_Information:
       GTModelTypeGeoKey (Short,1): ModelTypeProjected
       GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
       GTCitationGeoKey (Ascii,233): "IMAGINE GeoTIFF Support\nCopyright 1991 - 2001 by ERDAS, Inc. All Rights 
Reserved\n@(#)$RCSfile: egtf.c $ $Revision: 1.7 $ $Date: 2001/09/27 14:59:29EDT $\nProjection Name = Lambert Conformal 
Conic\nUnits = meters\nGeoTIFF Units = meters"
       GeographicTypeGeoKey (Short,1): User-Defined
       GeogCitationGeoKey (Ascii,259): "IMAGINE GeoTIFF Support\nCopyright 1991 - 2001 by ERDAS, Inc. All Rights 
Reserved\n@(#)$RCSfile: egtf.c $ $Revision: 1.7 $ $Date: 2001/09/27 14:59:29EDT $\nUnable to match Ellipsoid (Datum) to 
a GeographicTypeGeoKey value\nEllipsoid = Clarke 1880 IGN\nDatum = NTF"
       GeogGeodeticDatumGeoKey (Short,1): User-Defined
       GeogPrimeMeridianGeoKey (Short,1): PM_Greenwich
       GeogLinearUnitsGeoKey (Short,1): Linear_Meter
       GeogAngularUnitsGeoKey (Short,1): Angular_Degree
       GeogEllipsoidGeoKey (Short,1): User-Defined
       GeogSemiMajorAxisGeoKey (Double,1): 6378249.2
       GeogSemiMinorAxisGeoKey (Double,1): 6356515
       ProjectedCSTypeGeoKey (Short,1): User-Defined
       PCSCitationGeoKey (Ascii,190): "IMAGINE GeoTIFF Support\nCopyright 1991 - 2001 by ERDAS, Inc. All Rights 
Reserved\n@(#)$RCSfile: egtf.c $ $Revision: 1.7 $ $Date: 2001/09/27 14:59:29EDT $\nProjection = Lambert Conformal Conic"
       ProjectionGeoKey (Short,1): User-Defined
       ProjCoordTransGeoKey (Short,1): CT_LambertConfConic_2SP
       ProjLinearUnitsGeoKey (Short,1): Linear_Meter
       ProjStdParallel1GeoKey (Double,1): 45.8989194
       ProjStdParallel2GeoKey (Double,1): 47.6960139
       ProjNatOriginLongGeoKey (Double,1): 2.33722917
       ProjFalseOriginLongGeoKey (Double,1): 0
       ProjFalseOriginLatGeoKey (Double,1): 46.8
       ProjFalseOriginEastingGeoKey (Double,1): 600000
       ProjFalseOriginNorthingGeoKey (Double,1): 2200000
       End_Of_Keys.
    End_Of_Geotiff.

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    | Geospatial Programmer for Rent