Geographic Coordinate Systems

Geographic Coordinate Systems

Due to the complexity of modeling the geoid and the reasonable similarity between Earth and a regular ellipsoid, we do not use the geoid directly to designate horizontal locations on Earth’s surface. Instead, we use geographic datums—models that describe the locations on an approximation of Earth as a smooth, defined ellipsoid.

Horizontal datums denote locations using a system of longitude and latitude. The network of latitude and longitude lines that appears on a map is called the graticule. Horizontal datums are created using a reference ellipsoid – an ellipsoid whose shape approximates that of Earth’s surface. Not all datums use the same reference ellipsoid. Similar to how maps are designed with a purpose in mind, the specifics of a reference ellipsoid’s shape and position are determined based on the intended use of the datum.

The two illustrations in Figure 5.2.1 below demonstrate how datums differ in their design based on their intended purpose. In Figure 5.2.1 (left), the reference ellipsoid is aligned to closely fit the geoid in one part of the world (Australia). This is a local datum developed for use in Australia, and though the ellipsoid fits other parts of the world poorly, this is acceptable given the datum's intended use. In Figure 5.2.1 (right), the reference ellipsoid more closely fits the geoid overall. This is important for horizontal datums that are used to specify coordinates across the entire globe. The reference ellipsoid in 5.2.1 (right) is also centered at the center of Earth’s mass, which is important for GPS positioning.

Figure 5.2.1 The Australian Geodetic Datum (AGD84) (left), and the Geocentric Datum of Australia (GDA94) (right).

Credit: Intergovernmental Committee on Surveying and Mapping, Australia

Another type of datum is a vertical datum, which is used to specify vertical heights from a base surface approximated using calculations of mean sea level. Vertical datums are important for designing cartometric maps, but we will not discuss them in detail here. 

The three most popular datums used in North America are the North American Datum of 1927 (NAD27), the North American Datum of 1983 (NAD83), and the World Geodetic System (WGS84). NAD27 was the first standardized connected system of location points in North America. It was based off the Clarke Ellipsoid of 1866—measurements were made and recorded based on the relative positioning of all locations from Meade’s Ranch in Kansas. NAD83 is the modernized replacement of NAD27 and sought to improve positional accuracy as a result of adding thousands of new benchmarks. 

NAD83 replaced NAD27 in 1983; its increase in accuracy came from its use of a greater number of benchmarks than NAD27. Still, NAD83, still relied on the human measurement of triangulation from a control points. The World Geodetic System (WGS84) was developed alongside GPS technology, permitted the creation of an accurate worldwide datum. WGS84 is the standard datum used by GPS technologies today, though NAD83 remains popular for non-GPS-based mapping activities in North America. A new horizontal datum for North America, the North American Terrestrial Reference Frame 2022 (NATRF2022), is under development and is planned for release in 2025. More information on this and other new datums is available from the National Geodetic Survey.

Historical maps and data often reference the now-outdated NAD27 datum; it is important to be aware of the datum which was used to designate the locations of your spatial data. Datum transformation is the process of re-calculating locations based on a different datum and may be necessary if you are combining datasets that were specified using different datums (e.g., NAD27 vs. NAD83), or if you are hoping to map historical data using a more up-to-date system. 

As noted previously, modeling the earth as an ellipsoid or geoid is necessary for Cartometric mapping—mapping that involves the taking of precise measurements. Current GIS software tools (and the computers they run on) are now powerful enough to create projections based on an ellipsoidal Earth without much difficulty. For most thematic mapping purposes, however, conceptualizing Earth as a sphere is close enough. 

For the rest of this lesson, we will discuss Earth’s shape as if it were spherical, despite this being an oversimplification. The reason for this is that to create a map—that is, a 2D (flat) rendering of Earth’s surface—we need to represent a 3-Dimensional object on a 2-Dimensional plane. And even with a simple sphere, this is no simple task.