[gdal-dev] Is PAM always used before GeoTIFF tags?

Tue Mar 22 04:56:05 PDT 2022

Hi,

See this question https://gis.stackexchange.com/questions/426418/create-rectified-geotiff-given-aux-xml-produced-from-arcgis?

The GeoTIFF driver documentation https://gdal.org/drivers/raster/gtiff.html#georeferencing says about the order in which the georeferencing is searched "By default, information is fetched in following order (first listed is the most prioritary): PAM (Persistent Auxiliary metadata) .aux.xml sidecar file, INTERNAL (GeoTIFF keys and tags), TABFILE (.tab), WORLDFILE (.tfw, .tifw/.tiffw or .wld)."
However, in this case it seems that GeoTIFF tags are used instead of PAM if not especially asked with --config GDAL_GEOREF_SOURCES PAM

I am also curious to know how the SourceGCP values in this PAM file gets converted into pixel rows and columns. For example these values
<SourceGCPs xsi:type="typens:ArrayOfDouble">
        <Double>3.3670799903498199</Double>
        <Double>14.344873843184562</Double>
seem to turn into (1010.12399710495,3376.53784704463). How does it happen and where is it documented? I think there must happen some scaling and offsetting but is this something ESRI specific? I found this ESRI document but there the columns and rows appear unscaled in the aux.xml file https://doc.arcgis.com/en/imagery/workflows/browse-imagery/workflow/workflow-appendices.htm.

-Jukka Rahkonen-

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