[Geotiff] User defined datum: Transformation parameters

Frank Warmerdam warmerdam at pobox.com
Fri Feb 29 12:51:31 PST 2008

Christian Zietz wrote:
> Below is what I implemented now. I wrote some documentation so that
> other people facing the same problem can use this solution. The key ID
> was chosen at random but since there is no registry for geokeys I can't
> guarantee that it is not in use elsewhere.
> GeogToWGS84GeoKey
> KeyID = 35459
> Type = 7 * DOUBLE
> Values = dX, dY, dZ, Rx, Ry, Rz, dS
> This key allows the specification of a position vector 7 parameter
> transformation (as defined by EPSG:9606) between the coordinate
> reference system of the file and the WGS84 system. dX, dY and dZ define
> the translation vector and are given in meters. Rx, Ry and Rz define the
> rotation in seconds of an arc. dS is the scale correction expressed in
> parts per million.
> Note that there exists another 7 parameter transformation (the
> coordinate frame rotation, EPSG:9607) which differs from the method used
> here only in the signs of the rotation parameters Rx, Ry, Rz.
> Also the more simple 3 parameter transformation (geocentric translation,
> EPSG:9603) can be expressed with this geokey by simply setting Rx, Ry,
> Rz and dS to zero.


Would you consider switching to using 2062 for GeogToWGS84GeoKey?  This
puts it in the "geographic sc parameter keys" range.  I'm also leery about
using a 2^7 value for a geokey.

With that minor adjustment I'd be happy to post this as a technical note
on the geotiff web site, and also incorporate it in GDAL as well as add
support in the "normalize" machinery in libgeotiff for it.

It would be convenient if you could also supply a small sample file
demonstrating the new geokey.

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

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