Horizontal Datum Introduction

The datum represents a vital but frequently overlooked part of a map or digital map data. The datum consists of a starting point and orientation, and an assumed shape of the earth (ellipsoid).    For historical reasons a wide variety of local datums have been used in mapping.  NIMA's TR 8350.2 lists about 200 datums.  Some of the reasons for the multitude of datums include historical pride, but different datums best fit different parts of the world, and in the 19th century it was not easy to survey islands or colonial outposts relative to a known location and to tie regions together. Although most coordinate systems now use longitude relative to Greenwich, that was not usually the true starting point for the projection; the coordinates of the true starting point (Meades Ranch, Kansas, for the historic US datum of NAD 1927) were merely adjusted to put Greenwich on the prime meridian after its universal designation in 1884. With the advent of space based geodesy, we can now develop true earth centered datums that provide a best fit ellipsoid valid over the entire globe.

The most common global datum today is WGS84 (World Geodetic System), championed by NGA for world-wide mapping and the native reference for the GPS satellites.  For all practical purposes WGS84 is the same as NAD83 (North American Datum 1983), and they merely used different formal definitions which lead to miniscule differences, although the continued drift of the continents complicates precise measurements.   We recommend that you always use WGS84.  All NGA data, most USGS data, and current TIGER data, come in that datum.  The accepted European datum is ETRS89, which is also essentially the same as WGS84, but a number of the mapping agencies there still use their old national datums and projections.

In using digital data sets, you must be aware of the datum for several different things, and insure that they are correctly set:

  1. The datum used to digitize the map. GIS software should correctly identify the datum for both USGS and NGA data sets and most other data sets that have metadata.  World files and shape files will be notable exceptions because the formats do not include the projection and datum in their definitions, and the work-arouonds are not universal.
  2. The datum used for local paper maps if you will use the maps to manipulate the digital data, or compare them to the on screen data.  Paper maps and charts should show the datum in the legend or marginal information on the bottom of the map. Old U.S. maps that do not indicate the datum are almost certainly on NAD27 (or so old they might have not a datum). Datums affect both lat/long and UTM coordinates.
  3. GPS units report coordinates by default on the WGS84 datum, but you can transform the coordinates to another datum.  This might not be easy of a phone, which is replacing the more sophistiated and dedicated GPS units.

If these datums are not correctly set, screen coordinates will not match up with coordinates on the paper map. If the two datums are correctly set, data set accuracy and resolution may affect how closely features on the screen (like hill tops or valleys) match up with their true coordinates. It is often better to select the high point in the data set rather than to rigidly rely on hilltop coordinates read from a map.

GIS users who want the full power of the GIS to merge and combine different data sets must insure that they are on the same datum or have been shifted to lie on the same datum.

This US is getting a new set of datums, horizontal and vertical, in 2022.  It will have 4 reference frames, one for each plate with US Territory (North America, Pacific, Caribbean, and Marianas).

See also:

Last revision 1/19/2018