Datum

The Basic Concept of Datum

Before describing the meaning and role of a datum in the context of geographic information science, it is useful to illustrate the basic, explicit relationship between a datum (or frame of reference) and the coordinates used to define location.

Figure 1 shows a set of 2D Cartesian axes, labeled as the x-axis and y-axis. Also shown in the figure is a point, labeled P. The location or position of P can be expressed by assigning coordinates relative to the axes. In this case, P has coordinates (xP, yP), where

The Cartesian axes shown in Figure 1 provide a datum relative to which a unique location for any point can be given.

Now consider Figure 2. Once again, the location of point P can be expressed as (xP, yP) relative to the (x, y) Cartesian axes labeled as Datum A (solid lines). But this time, a second set of Cartesian axes (u, v) is shown and labeled as Datum B (dashed lines). Relative to this second frame of reference, the coordinates for P are (uP, vP). Notice that while there is only one point P, it now has two sets of coordinates. It is obvious from Figure 2 that both sets of coordinates are valid, but they have meaning and correctly define the location of P only when related to the correct datum. For example, to use the coordinates (uP, vP) to define the location of P relative to Datum A is wrong and will incorrectly locate P.

The important conclusion from this discussion is that coordinates are datum (or reference frame) dependent. To supply point coordinates and not specify the relevant datum results in an ambiguity that may lead to serious errors.

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