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In an earlier post I asked how to retrieve this tag from TIFF using python and libtiff. Eventually I found the answer and I reported on the solution last week. I also realised that in order to write a proper GeoTIFF, more tags are needed than only GDAL_NODATA, viz. ModelPixelScaleTag, ModelTiepointTag, ModelTransformationTag, GeoKeyDirectoryTag, GeoDoubleParamsTag, GeoAsciiParamsTag and GDAL_METADATA. Maybe even more are needed? The GeoTIFF format specification is found here - rather extensive ...

Now I want to also know how to write private tags. It goes wrong when I reach line 934 of module libtiff_ctypes: r = libtiff.TIFFSetField(self, tag, data)

The GDAL_NODATA tag is part of the dictionary tifftags - see my earlier post. The error message that is reported is: WindowsError: exception: access violation reading 0xFFFFD8F1. What am I doing wrong?

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With the help of a colleague, I have found the problem. This was my code:

tif.SetField("GDAL_NODATA", int(nodatavalue))

The definition of GDAL_NODATA I used was however like this:

TIFFFieldInfo(42113, -1, -1, TIFFDataType.TIFF_ASCII, FIELD_CUSTOM, True, False, "GDAL_NODATA")

The error is no more coming since we changed the first statement to this:

tif.SetField("GDAL_NODATA", str(nodatavalue))
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