Dictionary, Census of Population, 2021
Datum

Definition

A datum is a geodetic reference system that includes an ellipsoid (a mathematical reference model of the earth) and an origin against which the latitude and longitude of all other points on the earth's surface are referenced.

Reported in

2021, 2016, 2011, 2006 and 2001 (North American Datum of 1983)

1996, 1991, 1986, 1981, 1976 and 1971 (North American Datum of 1927)

Remarks

Datums and the coordinate reference systems based on them were developed to describe geographic positions for geodesy, navigation, surveying, geographic information systems, remote sensing and cartography. 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.

The most common datums used in Canada are the North American Datum of 1927 (NAD27), the North American Datum of 1983 (NAD83) and the World Geodetic System of 1984 (WGS84). The NAD27 is based on the Clarke ellipsoid of 1866, and its reference point is a fixed point in Kansas. The NAD83 and WGS84 are earth-centred datums based on a more accurate 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.

Because the ellipsoid differs between datums, along with their origins and orientation in space, 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, blockface 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, digital cartographic files, the road network file, blockface and enumeration area representative points, and other spatial data were based on the NAD27.