UTM (2)

Von Chuck Santose, Lt. Col., Armor, 18 Aug 95, CIS 76711,775 im CompuServe-Forum  

Grid Reference Systems: Reference: US Army Field Manual 21-26, Map Reading and Land Navigation, 7 May 1993. (Approved for public release; distribution is unlimited.)

Much of the following shamelessly plagiarized from the FM:

4-3. Military Grids. An examination of the transverse Mercator projections, used in large scale maps, shows most lines of latitude and longitude are curved lines and the quadrangles formed by their intersections are all of different shapes. This complicates the location of points and determination of direction. To aid these essential operations a rectangular grid is superimposed upon the projection.

This grid (a series of straight lines intersecting at right angles) furnishes the map reader with a system of squares similar to the block system of most city streets. The dimensions and orientation of different grids vary, but three properties are common to all military grid systems: (1) they are true rectangular grids, (2) they are superimposed on the geographic projection, and (3) they permit linear and angular measurements.

a. Universal Transverse Mercator Grid. The UTM grid has been designed to cover the area between 84N and 80S latitude and is imposed on the Mercator projection. There are 60 zones, each 6 wide and 164 tall; and each has it's origin at the equator. The grid is identical in all zones.

Base values (in meters) are assigned to the central meridian (of each of the 6 zones) and the equator and the grid lines are drawn at regular intervals parallel to these lines. With each grid line assigned a value denoting it's distance from the origin the problem of locating any point becomes progressively easier. This is usually done at 1000 meter intervals.

It would seem logical [!] to assign a value of 0 to the two base lines and measure distances outward from them, but this would require that a direction (N, E, W, or S) be always given with distances or that all points south of the equator or west of the central meridian have negative values. This inconvenience is eliminated by the use of "false values" to the base lines resulting in positive values for all points within each zone. This is 500,000 meters "false easting" for each central meridian and 10,000,000 meters "false northing" for the southern hemisphere.

b. Universal Polar Stereographic Grid. The UPS system is used in the polar regions (above 84N and 80S latitude). The "north south" base line is the 0/180 meridian and the "east west" base line is the two 90 meridians. For anyone traveling the tundra: 180 is "up" in the north polar region and 0 in the south. Grab a copy of TM 8358.1 if you've got more questions on this, it's too cold in these regions for me!

4-4. The US Army Military Grid Reference System is designed for use with both UTM and UPS grids. Either of these grids can include as many as 15 digits. The MGRS substitutes a single letter for several numbers and reduces the length.

a. Grid Zone Designation. The 6 UTM system results in 60 zones. The UTM Grid further breaks this down into zones 6 wide by 8 tall. The northernmost zone is 12 tall (it's mostly ice anyway!). This means there are 19 x 8 plus 1 x 12 tall zones covering the total of 164 from 80S to 84N. These are LETTERED starting in the south with C and ending in the north with X. I and O are omitted to avoid confusion with similar letters. Letters A, B, Y, and Z are used in the South and North polar regions in the UPS grid system where they form half circles rather than rectangles. GPS receivers using UTM show this letter as well as the UTM number.

b. 100,000 Meter Square. In the MGRS, each of the UTM rectangles (6 x8or x12) is further divided into 100,000 meter squares, each of which is identified by a two letter designation. This identification is unique within the area covered by the grid zone. They start with AA (easting northing) in the southwest corner. The square to the east is BA (easting northing). The square above AA is AB (easting northing). Same principle as UTM numbers. Right on Up.

In UTM, these 100,000 meter squares are numbered using the FALSE COORDINATES in lieu of the letters in MGRS. Keep in mind that "False Northing" in the northern hemisphere is 0 meters, so it's really true! The field manual example uses the notation in the lower left corner of the Columbus GA mapsheet (Hey, that's Fryar Field drop zone!). The lowest EW grid line on this mapsheet is numbered 3570000mN meaning it's 3,570,000 (3750 kilometers) meters north of the equator -- measure this if you're doubtful! The NS grid line is numbered 689000mE meaning it's 689,000 meters east of the false easting reference meridian or 189,000 meters east of the actual easting reference meridian (remember that 500,000 false easting?). This corner is also by happy convenience exactly 85W.

The 6 zone spans 84W to 90W so the central meridian is 87W confirming that the line is indeed east of the actual easting reference meridian. [Giving me a headache!]

Remember that 6 wide grid zones get narrower as they get away from the equator so that there are less 100,000 squares in the end zones than the center as the ones on the edges get pinched out.

Finally each 100,000 meter square is divided into squares 1000m x 1000m. This is really a convenience item to aid identifying them as the 100,000 meter square reference already provides identification of each position with in that square.

GPS UTM/UPS designations almost universally display data in the following format (using the Benning map from the FM): 16S 0689000 3570000

The 06 in the easting is shown on maps in smaller type then the 89000 as it's the 100,000 reference as is the 35 northing. The numbers most folks really need are 89000 and 70000 which are "1 meter" coordinates. The typical GPS will display this as if the position was accurate to 1 meter, which is something you can be absolutely sure is NOT the case!

The same location shown above in MGRS is 16SFL8900070000 to 1 meter precision but usually just FL890700 is all that's needed for most missions.

DOD maps come pre-gridded with both UTM and MGRS notations. Most other USG maps come without the lines, but do have blue tick marks in the margins showing where the UTM lines would have been had they not been too lazy to draw them. Use waterproof ink and a steel yard stick to draw them.

FM 21-26 is pretty mum on MAP DATUM, but users of GPS equipment should know that this is CRITICAL! Leaving your GPS set on factory default WGS-84 datum when your map was almost certainly surveyed using NAD-27 (or something else) will introduce massive errors into your navigation.

Errors at Fort Knox KY show positions 300+ meters north of their actual locations on the ground. Other locations will vary.

The computer program MADTRAN will convert coordinates from MGRS to UTM to Lat/Long and is public domain. Its available on several BBS including the Sailing Forum on Compuserve (GO SAILING).

You can order FM 21-26 from the Superintendent of Documents or some of the military stores. Beware of editions before 7 May 93 as they're grossly out of date.