[Proj] Creating a nadgrid shift file

Jan Hartmann j.l.h.hartmann at uva.nl
Thu Jan 17 12:10:40 EST 2013

(Sorry for crossposting, don't know if it is a gdal or proj problem )

This has been discussed on this list some time ago. I have been able to 
create a working nadgrid file with gdal, but it doesn't return the right 
coordinates. I did it as follows:

- create a 4 band, float32 geotif, each pixel with the displacements in 
lat, lon (arcseconds), errorx and errory (both zero)
- gdal-translate it to ntv2-format.

The displacements in the geotiff are correct, and proj returns shifted 
coordinates when using the ntv2-file, but not on the right location. I'm 
really out of my wits. Could it have something to do with ntv2 using 
negative lon-values east of Greenwich? Any help would be appreciated.

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