The bottom line for accurate massed fires: Common Grid

FA Journal, Jan-Feb, 2003

A datum can be local or global. A local datum covers only certain geographic regions. The North American Datum of 1927 (NAD27) is an example of a local datum and is still used in the United States, Canada and Mexico.

Global datums provide worldwide use. Examples include the World Geodetic System of 1972 (WGS72) and the newest, World Geodetic System of 1984 (WGS84).

The National Imagery and Mapping Agency (NIMA), Fort Belvoir, Virginia, considers the WGS84 the preferred datum, which is the datum most of us are familiar with today. NIMA predicts it will take up to 10 years to completely reference the world to the WGS84 Datum, but NIMA will only produce new topographic land maps (TLM) in the 1:100,000 scale.

Many of the existing 1:50,000 scale TLMs could remain referenced to one of several local datums still in use around the world. In some parts of the world, the accuracy error caused by using two different datums can be as much as 750 meters.

When combining the use of TLMs, digital maps or self-location systems, it is critical we know what datum we are operating on. Vertical datums are used as references for elevation; the most common is mean sea level (MSL).

The ellipsoid is basically a mathematical model for the size of the earth and is described as an oblate sphere: a sphere that is flattened at the poles. The ellipsoid was once called a "spheroid," and the term is still found on some of the older maps. There are more than five ellipsoids used around the world with WGS 84, again, being preferred.

The introduction of GPS technology in the late 1980s made WGS84 the preferred datum because GPS receivers compute all positions on WGS84 latitude and longitude and then convert them to display what datum and coordinate system the user needs.

Coordinate/Grid System. Another component of common grid is the coordinate/grid system. US forces use different coordinate systems. The Army and Marine Corps use the Universal Transverse Mercator (UTM) Grid and Military Grid Reference System (MGRS), while the US Navy uses latitude and longitude expressed in degrees, minutes and seconds. The Air Force uses latitude and longitude expressed in degrees and decimal degrees.

Imagine having an Air Force pilot checking in on station and requesting Army coordinates in latitude and longitude. Too many Field Artillerymen can't plot latitude and longitude on their maps or don't know enough about their handheld GPS receivers to convert coordinates into latitude and longitude. Today's joint operational environments require the warfighter be familiar with all these coordinate systems and know how to convert between the different formats. (See Figure 2.)

Map Projection. A map projection portrays all or part of the round earth on a flat surface. This cannot be done without some distortion; therefore, many different projections are used to produce maps. The most common projection is the Transverse Mercator Projection, the standard for NIMA-produced maps.

Many countries use other map projections unfamiliar to our forces that would necessitate their conversion. Fortunately, datum, ellipsoid, grid system and map projection information is found in the margins of all NIMA-produced TLMs. Digital maps and other digital products reference the WGS84 datum/ellipsoid and can be displayed in MGRS, UTM or Geographic grid coordinates using the joint mapping tool kit (JMTK) built into systems, such as the advanced Field Artillery tactical data system (AFATDS). Figure 3 gives the website for the NIMA geodetic translatot (GEOTRANS) free software to convert datums, ellipsoids, coordinates/grids and map projections.