A datum is a geodetic reference system which includes an ellipsoid and an origin against which the latitude and longitude of all other points on the earth's surface are referenced. A datum may often be associated with a particular ellipsoid (mathematical reference model of the earth).
2011, 2006, 2001 (North American Datum of 1983)
1996, 1991, 1986, 1981, 1976, 1971 (North American Datum of 1927)
Datums and the coordinate reference systems based on them were developed to describe geographic positions for surveying, mapping and navigation. Over the years, datums evolved from spherical to ellipsoidal models using satellite measurements. The earth is not a sphere, but an ellipsoid flattened slightly at the poles and bulging somewhat at the equator. The ellipsoid is used as a surface of reference for the mathematical model of the earth. Since mathematical models of the size and shape of the earth are now more precise, it has become necessary to change to a more accurate model.
There are two datums used in Canada: the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83). Both are geodetic reference systems, but each is based on different measurements and reference ellipsoids. The NAD27 is based on the Clarke ellipsoid of 1866, and its reference point is a fixed point in Kansas. The NAD83 is an earth-centred datum based on a newly defined ellipsoid – the Geodetic Reference System of 1980 (GRS80) – and its reference point is the centre of the earth, as opposed to a point on the earth's surface.
The National Transformation software, developed by the Geodetic Survey of Canada, is used to convert coordinates between the NAD27 and the NAD83 reference systems in Canada. Spatial data based on one datum will not be coincident with the same spatial data based on another datum. Positional differences between NAD27 and NAD83 can be as great as hundreds of metres in some instances. Other longitudinal analyses will also be affected by a change in datum. For example, block-face and census subdivision (CSD) representative points from censuses prior to 2001 may not fall in the correct current census standard geographic area due to a shift caused by using different datums.
Refer to the related definitions of cartographic boundary files (CBFs); coordinate system; digital boundary files (DBFs); map projection; representative point; road network file (RNF) and Spatial Data Infrastructure (SDI).
Changes Prior to the current census
Prior to 2001, digital boundary files (DBFs), digital cartographic files (DCFs), street network files (SNFs), block-face and EA representative points, and other spatial data were based on the NAD27.