Datums

Using spheroids and geoids, we can create a series of surfaces that combine these conceptual representations of the earth’s surface.   Why create a series of surfaces instead of just one? Geodesists create customized models of different regions of the earth’s surface by choosing a specific spheroid fit for purpose and vertically align it with the geoid in the regional extent of interest. We can achieve increased accuracy by customizing the shape of the spheroid and fitting it to the geoid for specific regions and extents. There are surfaces that have been developed for accurate mapping in North America, where the geoid is closely aligned or fit to a specific spheroidal shape (a specific set of major and minor axes measurements). Europe has specific surfaces that have been created from fitting spheroids with customized major and minor axes measurements to the gravity surface of Europe. Much of the spheroid is ignored, while only the part of the spheroid matching the extent of the area of interest (North America, for example) is utilized. There are also full spheroids fit to the geoid to create global surfaces. Remember, these customized surfaces are curved to represent the curve of the earth’s surface.

These defined curved surfaces are called datums and all have individual names. The most current North American surface is called the North American Datum of 1983 (NAD83). The most current European surface is called the European Datum 1950 (ED50). Datum is the word used to describe one of these surfaces that is created by fitting a specifically shaped spheroid to the geoid in a specific region.

The datums most often used in North America are:

  • NAD27 – North American Datum of 1927 based on the Clarke 1866 spheroid (Clarke 1866)
  • NAD83 – North American Datum of 1983 based on the Geodetic Reference System spheroid (GRS 1980)
  • WGS84 – World Geodetic System of 1984 based on the WGS 1984 spheroid

Often, datums are referred to as geographic coordinate systems. While this isn’t wholly accurate, it doesn’t cause many complications. Datums are only one part of what defines a geographic coordinate system as you will see in the section on geographic coordinate systems. Again, remember that datums are curved surfaces as are geographic coordinate systems.

So, while spheroids approximate the curved shape of the earth, datums provide a way to define the position of the spheroid relative to the earth. Older datums were defined in relation to a specific location on the earth’s surface because we were limited to manual surveying techniques. More recent datums are defined using satellite technology relative to the center of the earth.

Local versus Global Datums

Before geodesists were able to use satellite technology to make detailed measurements of the earth, datums were defined by surface measurements. These are called local datums. NAD 1927, a local datum, is based on the Clarke spheroid of 1866 and uses a series of survey measurements originating from a location considered to be near the center of the contiguous US, Meades Ranch, Kansas. Many USGS datasets continue to be stored in NAD 1927 as most of the topographic maps were (and are) published in NAD27. The European Datum 1950 is also a local datum.

Some current survey work is also created using a local datum. For example, when working in a remote area where it might not be possible to connect to an established coordinate system, an assumed coordinate system can be established with an arbitrary origin for XY measurements. Assumed coordinate systems can later be tied into a known coordinate system for use in a GIS.

Once satellites began measuring the earth as a whole body, it was possible to create a single global spheroid. The Geodetic Reference System (GRS) spheroid was established in 1980 and was used as the basis for a new North American Datum in 1983. The World Geodetic System (WGS) spheroid was published in 1984. When first released, these two geocentric datums were considered coincident (NAD 83 and WGS 84). But that is no longer the case because they are slowly drifting apart. NAD 83 is connected to the North American tectonic plate in order to keep the coordinate values true to North America over time while WGS 84 is tied to the International Terrestrial Reference Frame. According to ESRI (2010) the current coordinates of WGS 84 and NAD 83 are generally one to two meters apart.

The World Geodetic System 1984 is the datum used by the global positioning system and is the basis for coordinates collected on your cell phones.