A reference datum is a known and constant
surface which can be used to describe the location of unknown points. On Earth,
the normal reference datum is sea level.
All GTM 15 minute topographic maps in
circulation use the NAD-27 (North American Datum, 1927) referencing system
based on the Clarke ellipsoid of 1866. More recent GTM S maps use the
NAD-83 referencing system which is based on the GRS-80 ellipsoid.
The datum used is printed on the
front or back of the map. Although the reference ellipsoids used in the
NAD-27 and NAD-83 are different, the changes are slight on large-scale maps.