2.1 What Is a Datum?

datum a reference surface of Earth

ellipsoid a model of the rounded shape of Earth

geoid a model of Earth using mean sea level as a base

geodesy the science of measuring Earth’s shape

A datum is a reference surface, or model of Earth that is used for plotting locations anywhere on the actual surface of Earth. The datum represents the size and shape of Earth, which, contrary to popular belief, isn’t perfectly round. Earth is actually an ellipsoid (or spheroid), meaning that it’s larger at its center than it is at its poles. Think of taking a basketball and squeezing the top and bottom of the ball to compress it slightly and you’ll have a basic spheroid shape. However, Earth isn’t a smooth ellipsoid—gravitational forces affect different parts of Earth in different ways, causing some areas to be out of synch with the ellipsoid. Another model of Earth, called a geoid, places Earth’s surface at mean sea level to try and account for these differences (see Figure 2.1 to examine how the geoid and the ellipsoid stack up against each other, and against the variable topography of Earth). The science of measuring Earth’s shape to develop these kinds of models and reference surfaces is referred to as geodesy.

FIGURE 2.1 How the real-world Earth, the ellipsoid, and the geoid match up.

Developing a datum means creating a mathematical model against which to reference locations and coordinates. Thus, when you’re plotting a point, you have a system for referencing this location. The difficulty in mapping with datums arises because there isn’t just one datum to use for all measurements of Earth’s locations. In fact, there are hundreds of datums in use. Some datums are global and measure the entire world, while some are localized to particular continents or regions. These are some of the more common datums you’ll likely encounter with geospatial data:

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NAD27 the North American Datum of 1927

NAD83 the North American Datum of 1983

WGS84 the World Geodetic System of 1984 datum (used by the Global Positioning System)

datum transformation changing measurements from one datum to measurements in another datum

Measurements made with one datum don’t necessarily line up with measurements made from another datum. Problems arise if you take one set of data measured in one datum and your friend takes a second set of data measured from a different datum and you put both sets of data together. Both datasets refer to the same place, but they won’t match up because you and your friend are using different forms of reference for measurement. For instance, data measured from the NAD27 datum could be off by a couple hundred meters from data measured from the NAD83 datum. This problem is compounded by the availability of several local datums, or reference surfaces, designed for smaller geographic areas. With the measurement differences that may arise, the best strategy when you’re dealing with geospatial data is to keep everything in the same datum. A datum transformation is required to alter the measurements from one datum to another (for instance, changing the measurements made in NAD27 to NAD83). Datum transformation is a computational process, a standard one in many geospatial software packages. Datums can be used to set up geographic coordinate systems (GCS) for measuring coordinates.