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May 17 2011

Where am I?

“It is clear that the earth does not move, and that it does not lie elsewhere than at the center.”                 – Aristotle (one of first to estimate size of the earth, On the Heavens, p54)

We’ve talked a little about geodesy and reference ellipsoids but what is a datum? It’s pretty simple, actually. A datum is way of calculating the earth’s shape for the purpose of establishing coordinates. A datum is inherently in geographic (latitude and longitude) coordinates. The coordinates for a datum are all geodetic, meaning anywhere on the earth’s surface referenced to where it’s defining the center.  This is important to understand because a coordinate system is not this way. A coordinate system takes a datum and applies a projection system to it creating a highly accurate xyz system in a local not global area.

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An earth datum is defined by a reference ellipsoid and an axis of rotation. Here are the most common datums that we run into (although I cringe whenever I still see NAD27…):

  • North American Datum of 1927 (NAD27) uses the Clarke (1866) reference ellipsoid on a non geocentric (meaning the center of the earth is not its center)  axis of rotation.
  • North American Datum of 1983 (NAD83) uses the GRS80 reference ellipsoid on a geocentric axis of rotation.
  • World Geodetic System of 1984 (WGS 84) also uses the GRS80 reference ellipsoid but is still slightly different from NAD83 because its axis of rotation is defined differently. This datum is what GPS is based on.

Datums is defined by 8 constants: 3 for the origin and 3 for the orientation (axis of rotation), and 2 for the reference ellipsoid. NAD83 & WGS84 have the same reference ellipsoid but differ in the axis of rotation. NAD83 is built to be more accurate in North American while WGS84 is world system. NGS has a goal to revise NAD83 by 2018 but it will more likely be completed by 2022.

Sources:

“Geodetic Datum Overview” by Peter H. Dana

“Datums, Heights, and Geodesy” by Daniel Roman

“What is Datum?” from MapServer

“The State Plane Coordinate System” Part 1, Part 2, and Part 3 by Dan Doyle

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