Add Correct GNSS Altitude Description

[GBG]

Hi, EXIFTools is great!  Thanks!

FYI, the GPS Altitude tag reads 'Height above sea level'.  This is incorrect.  GNSS returns height above WGS84 ellipsoid.  See this link for more information on vertical datums:  https://vdatum.noaa.gov/docs/datums.html

[Phil Harvey]

Don't blame me.  Here is a quote from the Exif 2.32 specification:

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 GPSAltitude tag. The reference unit is meters. Note that this tag is BYTE type, unlike other reference tags.

- Phil