[Proj] NTV2 Grid Shift files and gdalwarp

[Jan Hartmann]

Hi all,

I've been experimenting with grid-shift files in ntv2 format to 
rubbersheet old maps. I created a working ntv2-file as described in

http://lists.maptools.org/pipermail/proj/2013-January/006539.html

and added an epsg definition for PROJ and PostGIS. The thing works with 
cs2cs and with vector transforms in PostGIS, but when I try to warp a 
raster file I get lots of errors like:

  Inverse grid shift iterator failed to converge.
pj_apply_gridshift(): failed to find a grid shift table for
                       location (6.7638472dW,53.0926086dN)
    tried: MIN03001A01.ntv2

and an output file with rubbish

The location is within the grid.

The source raster has epsg:28992, without the +towgs84 parameters but 
with the ntv2 file, the output raster has epsg:28992 with the towgs84 
numbers. Essentially, the ntv2 file contains the distances between the 
scanned map locations translated to an approximate position in 
epsg:28992, and the correct ones. I computed the grid with boundaries 
far outside the mapped regions, but this does not seem to make a difference.

Can anyone help?

Jan
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