Returns the altitude used as a reference: ◼If the reference is sea level and the altitude is above sea level, "0" is returned. ◼If the altitude is below sea level, a value of "1" is returned and the altitude is indicated as an absolute value in the EXIF_GPSAltitude tag.
The reference unit is meters.
Default value is empty string, which means "unknown". An unknown value is not written when saving EXIF values.
Note: To save GPS data, you must set EXIF_GPSVersionID to '2.2.0.0' (or use the const, GPS_Version_ID)