Leads: the essential elements of a 3-D geographic coordinate

Air & Space Power Journal, Spring, 2005 by John W. Dix

To build the most effective force for 2020, we must be fully joint: intellectually, operationally, organizationally, doctrinally, and technically.

--Joint Vision 2020

PRECISION WEAPONS, SUCH as the Joint Direct Attack Munition, depend on precise three-dimensional (3-D) coordinates to locate their targets. However, there is not a common format for transferring 3-D geographic coordinates across networks to precision weapon systems. This article proposes a simplified methodology for describing the 3-D geographic location of a target and its target location error (TLE). The system is called LEADS.

During the Vietnam War, close air support (CAS) required having an aircraft close to a target and often exposed to hostile fire. (1) Aircrews needed visual contact with the target or an offset reference point--ground forces could usually hear and see the aircraft involved.

Aircrews used the CAS nine-line briefing form (CAS-9) as a template for logically organizing information about the location of the target relative to ground features, the aircraft, and the location of ground forces. The briefing form is a fill-in-the-blank template. It specifies a target's location in terms of magnetic heading, offset, distance from the initial point, and battle point; geographic location, elevation above mean sea level (MSL), and description; as well as the location of friend-lies, egress routes, and the means to be used to designate the target, such as laser or white phosphorous. (2) The CAS-9 describes a target with sufficient detail for a clear identification that prevents confusing it with another target.

The CAS-9 has proven its effectiveness for locating targets relative to features on the ground; however, it does not satisfy the information needs of precision weapons that navigate using a global positioning system (GPS). These weapons seek precise geographic coordinates that are measured relative to the World Geodetic System (WGS)-84 Ellipsoid--an invisible but precisely located surface based on the signals of GPS satellites (fig. 1).

[FIGURE 1 OMITTED]

Currently, there is no standard template analogous to the CAS-9 for organizing essential information about precise 3-D geographic coordinates. LEADS is a template for organizing essential information about precise 3-D geographic coordinates. The following description of the LEADS template proposes methodology to remedy this shortfall and consists of five elements:

1. The L in LEADS refers to the latitude and longitude of a geographic coordinate. Although there are various coordinate formats, it is intended for use with coordinates measured in degrees, minutes, seconds, and decimal seconds.

2. The E in LEADS refers to elevation. With the advent of the GPS, a geographic coordinate may be measured either as a distance above MSL or height above ellipsoid (HAE), which is the distance above or below the WGS-84 Ellipsoid. Maps, charts, and aircraft altimeters measure elevations relative to MSL. GPS-guided precision weapons require elevations based on HAE as their vertical reference (fig. 2).

[FIGURE 2 OMITTED]

3. The A in LEADS refers to accuracy. The accuracy of a geographic coordinate is known as TLE and indicates the difference between the estimated location of an object and its actual geographic location. TLE is expressed in terms of a horizontal and vertical distance measured in meters or feet.

The most common method for reporting a geographic coordinate's accuracy uses horizontal and vertical error, plus a measure of the probability that they are within a stated distance from a geographic point. For example, imagine a soda can that has a marble suspended somewhere within it. We can't see the marble, hut we know it's somewhere in the can. If we assume the marble is suspended in the exact center of the can, the horizontal error would be the distance from the marble to the side of the can, and the vertical error would be the distance from the marble to the top and bottom of the can. Therefore, if a geographic coordinate is reported with an accuracy probability of 50 percent, a horizontal error of plus or minus 100 feet, and a vertical error of plus or minus 50 feet, there will be a 50-50 chance that the coordinate is inside a cylinder that has a radius of 100 feet and a height of 100 feet.

Suppose the accuracy of an image or map were plus or minus 100 feet; coordinates measured using that image or map could not be any more accurate than plus or minus 100 feet, or approximately the nearest one second of latitude and longitude. Therefore, if a geographic coordinate from a source with an accuracy of plus or minus 100 feet is used by a geographic information system (GIS) that shows all of its coordinates to a precision of plus or minus one-thousandth (0.001) of a second of latitude and longitude, it implies that the coordinate's accuracy is known to within two inches, when in fact it is only accurate to within 100 feet.

There is no perfectly precise geographic coordinate. GIS's that add decimal places to coordinates from inaccurate sources do not improve the coordinate's accuracy. It is a misleading and risky practice because it implies the geographic coordinate is more accurate than actually exists and could result in the geographic coordinate being unintentionally misused in an application that requires a truly precise coordinate.