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.

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