Datums and grids—what you don't know can kill you

Military Intelligence Professional Bulletin, Oct-Dec, 2002 by Richard J. Major Manning

Using a standard military map, a forward observer calls in grid coordinates for a fire mission. The field artillery battery receives the mission and the fire control crews initialize their state-of-the-art ballistic computer in accordance with the operations order (OPORD). The rounds impact 200 meters short of the target.

The problem was neither an inaccurate map nor a software problem in the ballistic computer. Rather the problem was human error.

Misuse of Datums and Grids

The misuse of datums and grids is a problem in our modern military. Many soldiers and leaders do not understand and are often mystified about how to employ datums (1) and grids properly. In the vignette above, the forward observer used coordinates generated from a current map with the North American Datum 1927 (NAD-27) while the fire control battery initialized their computers using the military standard World Geodetic System 1984 (WGS-84) datum. The basic problem is that datums are like different languages--without a translator they are incompatible with one another, in this case providing a different physical view of the earth for what appear to be identical grid coordinates.

Datum. As used here, "datum" is the base reference for an X, Y coordinate system. Because the earth is not a perfect sphere, we developed mathematical models for the accuracy of coordinates in specific geographic areas. Known as "local datums," they use an irregular ellipsoid model to obtain a "best fit" of the earth's surface for a particular region. Inconsistencies with local datums led to the development of the single worldwide system called WGS-84. This is the good news. The bad news is that it will be many years before all U.S. maps convert to this world geodetic system. As a result, multiple datums are normal on the battlefield and the military must exercise them in training.

Accuracy. The Department of Defense (DOD) requires the highest accuracy for precision navigation and delivery of long-range missiles and other guided munitions. WGS-84 is the DOD global datum and the military mapping standard for all future products. The National Imagery and Mapping Agency (NIMA) and Global Positioning System (GPS) use the WGS-84 datum as their standard. Datum information appears on NIMA map sheets in bold letters on the face or in the legend. OPORDs must clearly articulate which datum(s) are in use for an exercise or contingency. In Korea, the only datum permitted in the support of military operations is WGS-84, which prevents mixing with the local datum, significant positioning errors, and possible accidents. To ensure usage of WGS-84 products, users should destroy older map stock and put operational safeguards in place to prevent accidental use. Comparing a local datum to WGS-84 introduces significant error. Figure 1 compares four of the most common local datums with the WGS-84.

Difference Between NAD-83 and WGS-84

The difference between the two is insignificant for most applications because the shift is less than a meter and well within an acceptable error rate. Current NIMA military maps identify both NAD-83 and WGS-84 as the synonymous horizontally accurate datum for maps. With the proliferation of GPS and precision munitions, accurate use of maps and datums is more critical than ever. Leaders must challenge their soldiers to gain a basic understanding of datums and to train on their proper use. Tools and training aids are available to assist your unit to train safety and succeed on the battlefield. Figure 2 suggests some sources of additional information.

Figure 1

Comparison of Common Datum Systems

Datum   Location  Average Shift

ED-50   Europe    175 meters
Tokyo   Korea     755 meters
NAD-27  U.S.      208 meters
NAD-83  U.S.      <1 meter

Figure 2

Sources of Additional Information

1. Datum Conversion Software: Geographic Translation Version 2.0
(GEOTRANS 2.0) is an easy to use Windows[R] (95 and NT) and UNIX
software used to convert local datums to WGS-84 format. GEOTRANS 2.0
is the DOD-approved datum transformation and coordinate conversion
program. Readers may download GEOTRANS 2.0 at the following website
http://164.214.2.59/GandG/geotrans/geotrans.html.

2. Grid and Datum Manual: This NIMA reference manual provides
information on grids and datums applied to DOD maps and charts. You may
download it at http://164.214.2.59/GandG/tm83581/toc.htm.

3. GPS Tutorial: This is a basic, nontechnical course of instruction on
CD-ROM (compact disc read-only memory) intended for Army GPS users. It
contains an emulator for students to become familiar with the PLUGGER
interface. The NSN is 7644014454559 and the NIMA Reference Number is GPS
XXTUTORCD. Please see the website at
http://www.wood.army.mil/TVC/FactSheets/gpsfctsht.htm.

Endnote:

(1.) In general writing, we use "data" as both singular and plural (the meaning changes based on whether data appears with a singular or plural verb form); "datum" is rarely used. However, this article uses "datum" in the specialized meaning employed in surveying and latitude and longitude as used by the Department of Defense and the National Imagery and Mapping Agency.