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"Exif.GPSInfo.GPSAltitude: Indicates the altitude based on the
reference in GPSAltitudeRef. Altitude is expressed as one RATIONAL
value. The reference unit is meters."

Since the value is stored as an _unsigned_ rational, it can't hold a
negative value.

"Exif.GPSInfo.GPSAltitudeRef: Indicates the altitude used as the
reference altitude. If the reference is sea level and the altitude is
above sea level, 0 is given. If the altitude is below sea level, a
value of 1 is given and the altitude is indicated as an absolute value
in the GSPAltitude tag. The reference unit is meters. Note that this
tag is BYTE type, unlike other reference tags."

Andreas