Simply put,a datum is the mathematical model of the Earth we use to calculate the coordinates on any map, chart, or survey system.

All coordinates reference some particular set of numbers for the size and shape of the Earth. For details on how the world is mapped click here:

http://164.214.2.59/GandG/geolay/toc.htm

The problem for warfighters is that many countries use their own datum when they make their maps and surveys--what we call local datums. Other nations' maps often use coordinates computed assuming the Earth is a completely different size and shape from what the Department of Defense uses, but we have to be ready to fight around the world!

US forces now use a datum called World Geodetic System 1984, or WGS 84. The Defense Mapping Agency (DMA)[now called National Imagery and Mapping Agency--NIMA] produces all of its new maps with this system. Unfortunately, we reprint many of our maps from products made by allied countries that use local datums. Our old maps were made on several different local datums, or sometimes WGS 72 (maps using this datum were often printed "World Geodetic System" with no year identification). So the old maps we're reproducing, and the foreign ones we reprint, might use those other datums.

"Why should I care?"

The coordinates for a point on the Earth's surface in one datum will not match the coordinates from another datum for that same point! A grid shift exists between datums because each datum has a different origin. Grid shifts may be relatively small, or in the case of Korea, very large. You may encounter a second problem when you plot Military Grid Reference System (MGRS) coordinates. There are two possible schemes for "1OO,OOOm Square Identifiers" (the XE in 51S XE 1234 5678). These letters change when datums are converted to WGS 84 in Africa and Asia, or North American Datum 1983 (NAD 83) in North America. Take the example above right: The datum of the top map is WGS 84. The coordinate for the

buildings circled in red is 51S XE 2818 1402. The datum of the lower map is Tokyo Datum. The coordinate for the same set of buildings is 51S XQ 2834 1331. Total grid shift in this example is 729 meters (+ I56mE. -7J2mN). PLUS the change in 1OO,OOO m square identifiers. You can see the problem in passing target coordinates if you are using a different datum than your supporting artillery.

"How can I check my datum?"

All of the maps and charts DMA distributes will have the datum printed somewhere in the margin like this:

Ellipsoid	World Geodetic System 1984
Grid	1,000 Meter UTM Zone 51S
Projection	Transverse Mercator
Vertical Datum	Mean Sea Level
Horizontal Datum	World Geodetic System 1984
Hydrographic Datum	Approximate level of lowest low water
Printed by	DMAHTC 4-49

And the UTM Grid Zone, 100,000 m square identifier map box shows the proper geo coordinates:

COORDINATE CONVERSION WGS 84 TO TOKYO

Grid. Add 158m.E.; Subtract 112m.N.

Geographic: Add 7.0" Long.; Subtract 8.8 lat

Notice there is a "Horizontal Datum" for location and a "Vertical Datum" for elevation. Almost all maps and charts use mean sea level for elevation, but they might use any of over a hundred different horizontal position datums. Sometimes, a map will have more than one grid on it. Normally, each grid is for a different datum!

Maybe. GPS receivers usually give you the option to read your coordinates in several datums, including WGS 84. One new unit now coming into use, the Precise Lightweight GPS Receiver or PLGR", allows you to use 49 predefined datums and two "user datums". The datums already programmed in are some of the ones you're most likely to see now on maps. If US forces go to a place where the maps are on some datum that the PLGR doesn't have, your command will get the figures for you to put in as the User Datum. Consult your GPS manuals to find out how to switch datums. But be careful; GPS coordinates are almost always more accurate than the ones you read off a map. The specification for the 1:50,000 Topo Line Map says objects are plotted on the sheet to within 50 meters of their true position. And you could add 50 more meters error just in reading the point. That's why maps should never be used for very precise targeting!

The bottom line is this: In the past, we didn't worry much about datums because our weapons usually didn't need highly accurate positions. But nowadays, precise coordinates are vital for mission success. Ignoring the fine print in the margin of a map could get you killed!

· Every higher headquarters staff has a Mapping Charting and Geodesy (MC&G) Officer.

He or she can help you find out which datums are in use in your AOR, and help you make sure you re using the right one. The unit that orders your maps should be able to tell you the MC&G Officer¹s number. If you can't find it, call one of the numbers below for help.

· DMA (NIMA) has Action Officers who track the status of maps in each potential region where US forces may be deployed. They work in the Plans and Requirements Directorate at DMA (NIMA) Headquarters, and can be reached at DSN 356-9329 or (703) 285-9329 Commercial.

The Defense Mapping School (DMS) will provide you with plenty of assistance. Mobile training teams can come to your location anywhere in the world at DMA (NIMA) expense to get you on the right track. Call DMA (NIMA) at DSN 655-3206 or (703) 805-3206 Commercial.

· DMA publishes software called MADTRAN which can convert coordinates from latitudel longitude to UTM or MG RS, and the reverse. It can also transform coordinates between WGS 84 and over a hundred other datums. MADTRAN will work on any IBM PC-compatible computer with a 5 1/4" floppy drive. It comes with instructions right on the disk, and is easy to use. MADTRAN 5.0 is the current version, and you can order it wherever you get your Defense Mapping Agency (NIMA) products. The stock number is MADTRANIBMPC.

This pamphlet covers coordinates we get from maps, but for very precise targeting, maps just aren't good enough. DMA (NIMA)distributes a poster that explains how to check the coordinates used for precise targeting. The DMA (NIMA) stock number is DIAXXCOORDGRAPH.