[gdal-dev] Questions about Raster Attribute Tables

[jratike80]

Mikael Rittri wrote
> It would seem that the .aux.xml format was designed by GDAL developers,
> although I haven't found any documentation on the history of the format.
> Esri documentation says: "ArcGIS 9.2 introduced the AUX.XML file for
> certain file formats", which can be understood as saying the format was
> invented by Esri, but I suspect they really mean something like "ArcGIS
> 9.2 adopted the AUX.XML format that was introduced by GDAL Library...". 
> Any comments on that?
> 
> The files named Filename.aux are a mystery to me. Esri documentation says:
> "The information stored in an AUX file is only accessible using a product
> from Esri, ERDAS, or a third-party product derived from the RDO/ERaster
> library." But I have found examples of datasets for which GDAL Library can
> retrieve a Raster Attribute Table from a binary Filename.aux. So, maybe
> some developer of GDAL Library has reverse-engineered the Esri
> Filename.aux format, but I haven't found any documentation of that. Or I
> guess maybe there are two distinct file formats that use the ".aux" file
> extension - the name "aux" isn't very creative and maybe Esri never
> bothered to trademark it.  Does anyone know?

Hi,

Origin is not ESRI but ERDAS. ESRI used a raster engine from ERDAS before
starting to use GDAL in version 9.2.

From
http://community.hexagongeospatial.com/t5/ERDAS-IMAGINE-Q-A/AUX-File-aux/ta-p/2286

The .aux extension indicates an auxiliary file, mainly used in conjunction
with non-native ERDAS IMAGINE files accessed by the Raster DLL, to store
information not normally supported by the particular raster format. The TIFF
file format, for example, does not store internal image statistics, so
statistics generated by ERDAS IMAGINE are stored in an .aux file with the
same prefix name as the .tif file.

GDAL supports ERDAS .aux files at least in HFA driver
https://www.gdal.org/frmt_hfa.html. 

-Jukka Rahkonen-





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