[gdal-dev] Troubles with PAM file

[Dominique Lyszczarz]

Hi all,

I'm trying to read a raster georeferenced with ArcGIS. It's a jpg file
coming with a PAM file (.aux.xml) that store GCPs and define a polynomial
transformation of order 3. Unfortunately GDAL does not take the PAM file
into account and gdalinfo does not report GCPs. After some investigations,
I found this commit
<https://github.com/OSGeo/gdal/commit/f8822eb50466fb740e228b8c6a7f066b916faa0a>
that suggest PAM files are supported by GDAL but only for TIFF dataset. So,
I've translated my file to TIFF and now gdalinfo report the GCPs but the
file is not correctly georeferenced. Reading more carefully the commit
previously cited I can see some relevants clues : sources GPCs are
considered to be expressed in inch and so the code rely on image dpi stored
in TIFF tags to compute the pixels coordinates. Futhermore the origin of
pixels coordinates is considered to be the bottom left. I've checked my PAM
file and clearly the sources GPC are expressed directly in pixel, not inch,
with an image origin fixed to top left. I can set the dpi of my image to 1
to fix the scale but Y coordinates remains wrong.

I'm a bit confused with theses PAM files, is there any technical doc from
ESRI? My PAM file make sense for me : pixel coordinates and top left origin
(like worldfile) for sources GPC, it's seems a very common approach. But
inch !? Also is there any particular reason to limit PAM file reading to
TIFF dataset ? maybe is it related to the need to extract dpi from tags ?

For now I can't see any way to correctly read this file with GDAL, so I'm
thinking of writing my own script to parse GCPs and rectify the raster wih
gdalwarp. Not sure if it will be the best approach here, any advices?

The file can be downloaded here :
https://drive.google.com/drive/folders/1hZXncJhcfZI0Uem-oCfUsruO9mPwWD0U?usp=sharing

Thanks in advance for any feedback on this.

Regards
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