Geographic Coordinate Systems

Ellipsoids and geoids are both ways to model the Earth, and thus there are multiple ways to “fit” these models to our physical planet. We do this by choosing a set of reference points, and using these reference points create a geodetic network called a datum. There are many different datums from which to choose. Each datum can be composed of unique ellipsoid and geoid models.

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 based primarily on a specified reference ellipsoid. We have already addressed the idea of a vertical datum, which is used to specify heights from a zero-surface (i.e. the geoid). Both horizontal and vertical datums are important for designing cartometric maps, as was mentioned in Types of Maps. But in order to get the most accurate horizontal measurements of a particular area of concern, one must select a horizontal datum that accurately models the geographic location in question.

The two illustrations in Figure 3.2.1 below demonstrate how datums differ in their design based on their intended purpose. On the 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. On the right, the reference ellipsoid more closely fits the geoid overall. This is a geocentric datum, which is ideal for global mapping projects. The reference ellipsoid on the right is also centered at the center of Earth’s mass, which is important for GPS positioning.

Figure 3.2.1: The Australian Geodetic Datum (AGD84) (left), and the Geocentric Datum of Australia (GDA94) (right).

Credit: Intergovernmental Committee on Surveying and Mapping, Australia(link is external)

The three most popular horizontal 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), which is actually considered to be a terrestrial reference frame (which expresses both horizontal and vertical datums all rolled into one standard. NAD27 was the first standardized connected system of location points in North America. It was based on 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 the addition of more benchmarks compared to NAD27. Still, NAD83, relied on human measurement of triangulation from control points. The National Geodetic Survey (NGS) has been working to replace NAD83 with a terrestrial reference frame that will encompass the entirety of the United States and its territories. Known as the North American Terrestrial Reference Frame of 2022 (NATRF22), this will combine the geometric and geopotential aspects into a single product that will rely primarily on Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), as well as on a gravimetric geoid model resulting from NGS’ Gravity for the Redefinition of the American Vertical Datum (GRAV-D) Project. The intended accuracy of heights relative to MSL in the geopotential component of NATRF2022 will be 2 centimeters over any distance, where possible. The magnitude of change with NATRF2022 will vary depending on your geographic location. NATRF2022 will change latitude and longitude positioning and ellipsoid height between 1 and 4 meters. In the conterminous United States (CONUS), NATRF2022 will change heights on average 50 centimeters, with approximately a 1-meter tilt towards the Pacific Northwest.

The World Geodetic System (WGS84), designed by the National Geospatial-Intelligence Agency, developed alongside GPS technology, was the first datum suitable for general worldwide use. WGS84 is the standard datum used by GPS technologies today, though NAD83 remains popular for non-GPS-based mapping activities in North America. As mentioned, a new geometric and geopotential datum for North America, NATRF2022, will soon be realized and available for use. More information on this and other new datums is available from the National Geodetic Survey(link is external).

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 coordinate locations or heights 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 from maps. 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 two-dimensional (flat) rendering of Earth’s surface—we need to represent a three-dimensional object on a two-dimensional plane. And even with a simple sphere, this is no simple task. In addition, the thematic maps that are created in this class do not have a high accuracy measurement requirement; thus, a spherical Earth assumption is sufficient for thematic map purposes.