[Proj] Using a GDAL-generated NTV2-file

Jan Hartmann j.l.h.hartmann at uva.nl
Tue May 24 04:51:39 EST 2011

On 05/21/11 18:06, Frank Warmerdam wrote:
> Jan,
> I believe the problem is that the test.tif is south up.  The NTv2 reader
> in PROJ determines the size based on the extents and pixel size and gets
> mixed up as the extents are upside down.  You can flip test.tif with
> gdalwarp.
> eg.
> gdalwarp test.tif northup.tif
Yes, now it works. Just to be sure: is the result of the flipping 
correct? The tif file was created from two point lists, each with a 
lat-lon coordinate and the displacement in arcseconds. Gdal_grid 
converted the lists into two grid files, with the upper boundary having 
a smaller latitude than the lower coordinate. That's OK, as latitude 
values decrease when going north, in contrast with projected coordinates.

Now, what happens when I use gdalwarp on this file? Is the complete file 
mirrored? Are the displacements at the right geographical location?

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