Question: 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.
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.
Formerly a Mapping Center Ask a Cartographer Q & A.