[gdal-dev] Rectify TIFF: Difference between applied GCP and warped image

[Jukka Rahkonen]

Bischof, Richard <Richard.Bischof <at> lgln.niedersachsen.de> writes:

> 
> Hi Jukka,
> 
> you're correct. ArcMap is doing the on-the-fly transformation with the
dataset, I applied the gcps using
> gdal_translate to.
> With both gdal_warp and ArcMap Rectify I use second order polynomial.
> 
> I also found, that some areas of my source image are cut out from the
gdal_warp destination dataset.
> 

Hi,

I can't really help you but because I do not understand warping algorithms.
If ArcMap and GDAL makes different output with 2nd order polynomial and with
the same gcp set I can see three alternatives:

1) GDAL is wrong
2) ArcMap is wrong
3) There are different interpretations about what should happen and both are
right, or wrong.

I suppose that what GDAL is doing is written in
https://trac.osgeo.org/gdal/browser/trunk/gdal/alg/gdaltransformer.cpp
I fear we do not have the code used by ArcMap available for making a comparison.

Can you simplify the case into a question like:
With this gcp set, applied to an image of sixe xxx(W) by yyy(H) pixels,
after 2nd order polynomial transformation with GDAL the source pixel (x1,
y1) is moved into location (x2, y2) in pixel space, and (xxx(E), yyy(N)) in
projected cooordinates in EPSG:xxxx.

ArcMap moves this pixel into (x2', y2') (xxx'(E), yyy'(N)) and I think than
one or the other is wrong. What do warping specialists think?

-Jukka Rahkonen-