Method returns the tag value if the tag is present in the metadata tree, or null otherwise.

This method accesses the GPSAltitudeRef tag. The tag 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 GPSAltitude tag.