This is a problem only when saving to XMP files.
What actually happens is this:
The negative altitude is correctly saved to the XMP file.
However reading negativ altitudes from XMP files does not work.
Exiftool retruns the absolute value when queried about XMP:GPSAltitude
Using the composite tag GPSAlitude returns the correct value.
Will be fixed in the next release of Geotag.
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This is a problem only when saving to XMP files.
What actually happens is this:
The negative altitude is correctly saved to the XMP file.
However reading negativ altitudes from XMP files does not work.
Exiftool retruns the absolute value when queried about XMP:GPSAltitude
Using the composite tag GPSAlitude returns the correct value.
Will be fixed in the next release of Geotag.
This fix has now been deployed in release 0.079