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WGS84 vs NAD83

April 29 2009 | 2 comments
Categories: Map Data

What is the difference between the WGS84 and NAD83 datums? If I create a map using the WGS84 datum, can I label the map as both WGS84 and NAD83? Or, does it depend on what scale the map is in?

I thought I heard that they are no longer interchangeable, but I recently saw a map labeled as WGS84/NAD83 for its horizontal datum. Thanks for any clarification.

Mapping Center Answer:

There are a number of difference between the NAD83 and the WGS84 datum. One is the reference ellipsoid. The North American 1983 datum (NAD83) uses the Geodetic Reference System (GRS80) ellipsoid while the World Geodetic System of 1984 (WGS84) uses the WGS 84 ellipsoid. The dimensions of these ellipsoids differ slightly. For more information, refer to Projection basics the GIS professional needs to know.

A map will have only one coordinate system, either Geographic or Projected in our software's terminology. For example, the "WGS84 projection" is a geographic one. A UTM projection is a projected one. Either of these will use only one datum. However, the data on the map could have come from multiple sources, all with unique projections and therefore datums.

The map that you saw could not possibly be drawn using both the WGS84 and NAD83 datums. That said, I note that some GPS data are self-described as "NAD83/WGS84" using the disclaimer that "The differences between these two datums for North America is not discernible with mapping/GIS grade or consumer grade GPS equipment." That may be true, but the cartographer would do further research to find out more. For example, here is one explanation I found: "For the sake of discussion, whenever you hear WGS84/NAD83, you can automatically assume it is NAD83. In this document we will refer to either WGS84/NAD83 or NAD83 as WGS84/NAD83". The cartographer should then know to make the note on the map clear that the map's projection (assuming it is the same one as the GPS data) really uses the NAD83 datum. If it is not the same one as the GPS data, the datum definition is embedded in the projection definition.

Look for a blog entry on this in the next day or two where I hope to make these distinctions clearer.

WGS84 (original) and NAD83 (1986) . . . posted by John Hutchinson on Jan 14 2013 11:46AM
. . . are essentially identical, according to this article in the March, 2000 issue of Professional Surveyor, authored by Richard A. Snay and Tomas Soler of the National Geodetic Survey: http://www.profsurv.com/magazine/article.aspx?i=561
(See the final paragraph)

Unless you're working at (I've read) 1:5,000 scale or larger, where the difference of a meter in ground position is significant, then you can redefine data in WGS84 (original) as being in NAD83 (1986) and vice versa as needed. ESRI, do you disagree with that statement?
It depends on time and place posted by Aileen Buckley on Jan 14 2013 3:12PM
I cannot speak for all of Esri, but I have consulted with our geodesist on the projections team. Here is what I think we can safely and accurately say:

If you are specifically talking about WGS 84 (original) and NAD 83 (1986) [aka the original], we agree. But you should realize that a lot of data is not on either of these reference frames. So the statement doesn’t hold true for later realizations of NAD 83 or WGS 84 which are NAD 83 (2011) and WGS 84 (G1674). And the variations between the later realizations of WGS 84 and NAD 83 vary by location as well as over time.

Here is an excerpt from Dr. Jon Kimerling from publication in progress:

"Since the WGS 84 geographic coordinate system was designed for worldwide positioning and mapping from GPS satellite da5ta, its datum and terrestrial reference frame could not be “locked in place” on a particular tectonic plate like the NAD 83 system is on the North American Plate. Instead, geodesists analyze the relative rate of movement of the tectonic plates on the earth’s surface and define an international reference pole, international reference meridian, and international reference equator that remain stationary relative to the average movement of the plates. These are very close to the geographic north Pole, the Greenwich Prime Meridian, and the geographic Equator, but change slightly in position over time.

Holding the reference pole, meridian, and equator stationary relative to the average movement of all tectonic plates means that the WGS 84 geodetic latitude and longitude of any location on earth is constantly changing primarily due to the motion of the location’s tectonic plate. The velocity vectors in figure 1.14 give you an idea of how tectonic plates are moving relative to the international reference meridian and equator. In England near the international reference meridian, for example, all features were moving in 2005 at a rate of about 2.5 cm (one inch) per year in a northeasterly direction. This movement translates into WGS 84 geodetic latitudes and longitudes increasing buy about 0.0005 and 0.001 seconds per year, respectively. This change in geodetic coordinates is small, but in places like Hawaii or Australia the rate of movement is about one meter per decade. Corresponding shifts in this magnitude of geodetic latitude and longitude muse be adjusted for in the WGS 84 system since3 survey-grade GPS receivers and other modern positioning technology can locate features to a centimeter accuracy. Periodic worldwide adjustments of WGS 84 geodetic latitude and longitudes due to tectonic plate motion are made by using International Terrestrial Reference Frame (ITFR) position updates."

Therefore, originally, WGS84 (original) and NAD83 (1986) could have be considered equivalent, particularly to the meter-level. Most data is no longer on either of these reference frames though, because of the ITRF updates described above.

To learn more, here is a link to an article from the National Geodetic Survey: http://www.ngs.noaa.gov/CORS/Articles/WGS84NAD83.pdf

Also, here is a link to the article "Using the HTDP Software to Transform Spatial Coordinates Across Time and Between Reference Frames" that talk about how ITRF realizations are changing over time and how WGS 84 is connected to various ITRF realizations. You will also see statements like a particular WGS 84 realization is within 2 cm of a particular ITRF realization: http://www.ngs.noaa.gov/TOOLS/Htdp/Using_HTDP.pdf

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