Natural Heritage Information Centre
The UTM coordinates of a point are usually determined from a grid printed on
the Federal NTS and on the OBM series of maps. The position and numbering of
the grid on the map sheet is determined on the basis of the position of the
map relative to a geodetic datum. Many maps are based upon a datum known as
the North American Datum 1927 (NAD27). There has been a revision to the datum
(known as the 1983 datum (NAD83)), leading to a revised grid on newer maps.
This could result in discrepancies of up to 200m between the two versions.
The datum is shown on the bottom centre of the map you are using. If the datum
is taken from a GPS, the WGS 72 is equivalent to NAD 27 and the WGS 84 is
equivalent to NAD 83.
Eventually, all maps will be based upon the NAD 83 and all coordinates will
be consistent. In the interim, however, it is important to record the DATUM on
which the coordinates were developed.