An extensive application of the transverse Mercator projection is in a grid reference system for military maps called the universal transverse Mercator (UTM) military grid system. In this system a reference plane grid, like those used in our state grid systems, is imposed on transverse Mercator projections of relatively small areas. The basic

Figure 9-14.(A) Grid zone designations of the military grid reference system; (B) 100,000-meter-square designations in the UTM military grid system,

Starting at the 180th meridian and progressing eastward by the compass, the earths surface is divided into a succession of north-south zones, each extending for 6 degrees of longitude. These zones are numbered from 1 through 60. Between latitude 80S and 84N, each zone is divided into a succession of east-west rows, each containing 8 degrees of latitude, with the exception of the northernmost row, which contains 12 degrees of latitude. Rows are designated by the letters C through X, with the letters I and O omitted. The lettering system begins at the southernmost row and proceeds north. For a particular zone-row area, the designation consists of first, the zone number and next, the row letter, such as 16S, which means row S in zone 16.

The polar regions (that is, the areas above 84N and below 80S) have only two zones in each area. These lie on either side of the 0-degrees and 180- degrees meridian. In the North Polar region, the half of the region that contains the west longitudes is zone Y; that containing the east longitudes is zone Z. No numbers are used with these designations. Similarly, in the South Polar region, the half containing the west longitudes is zone A; that containing the east longitudes, zone B.

In the UTM Military Grid System, a particular point on the earth is further identified by the 100,000- meter square in which it happens to lie. Each of the 6-degree longitude by 8-degree latitude zone-row areas in the system is subdivided into squares measuring 100,000 meters on each side. Each north-south column of 100,000-meter squares is identified by letter as follows. Beginning at the 180th meridian and proceeding eastward, you will find six columns of full squares in each 6-degree zone. Besides the full columns, usually partial columns also run along the zone meridians. The partial columns and full columns in the first three zones are lettered from A through Z, again with the letters I and O omitted. In the next time zones, the lettering systems begins over again. Observe, for example, figure 9-14, view B. This figure shows the zone-row areas in 1N, 2N, and 3N, and 1P, 2P, and 3P. The zone meridians shown are 180W, 174W, 168W, and 162W; the zone-row parallels shown are the equator (0 latitude), 8N, and 16N. The first 100,000-meter-square column to the east of 180 degrees is the partial column A. Next comes six full columns: B, C, D, E, F, and G. Then comes partial column H, to the west of the zone meridian 174W. The first column to the east of zone meridian 174W is partial column J; then comes the full-size columns K, L, M, N, P, and Q, followed by partial column R. To the east of zone meridian 168W,A through V, again with I and O omitted. For columns in the odd-numbered zones, the first row of squares north of the equator has the letter designation A; for columns in the even-numbered zones, the first row of squares north of the equator has the letter designation F. Rows above and below this row are designated alphabetically. The first row south of the equator in the odd-numbered zones, for example, has the letter designation V, while the first row south of the equator in the even-numbered zones has the letter designation E.

The complete designation for a particular 100,000-meter square consists of the number-letter, zone-row designation plus the two-letter, 100,000- meter-square designation. For example, the designation 1NBA means the first full square east of the 180th meridian and north of the equator (square BA) in zone-row 1N, as shown in figure 9-14, view B. If you know the latitude and longitude of a certain point on the earth, you can determine the designation of the 100,000-meter square in which the point lies. Take Fort Knox, Kentucky, for example, which lies approximately at latitude longitude

The location of a particular point within a 100,000-meter square is given by naming the grid coordinates of the 100-meter square (or, for more precise location, of the 10-meter square) in which the point lies. Within each zone the point of origin for measuring these coordinates is the point of intersection between the zone central meridian and the equator. A false easting of 500,000 meters, instead of a value of O meters, is assigned to the central meridian to avoid the use of west or negative east-west coordinates. For points in the earths Southern Hemisphere, the equator is assigned a false northing of 10,000,000 meters to avoid the use of south or negative north-south coordinates, and northing values decrease from the equator toward the South Pole. For points in the Northern Hemisphere, the equator has a coordinate value of 0 meters, and northing values increase toward the North Pole.

This procedure results in very large coordinate values when the coordinates are referenced to the point of origin. For example, for the bullion depository at Fort Knox, Kentucky, the coordinates of the lo-meter square in which the depository is located are casting 590,990 meters, northing 4,193,150 meters; however, since the grid zone-row designation pins the coordinate down to a relatively small area some of the digits of the coordinates are often omitted. Consider, for example, the part of a map shown in figure 9-16. The grid squares on this map measure 1,000 meters on each side. Note that the casting grid lines are identified by printed coordinates in which only the principal digits are shown, and of these, even the initial number 5 is in small type. The understood value of the number 589 is 589,000 meters. In setting down the coordinate for this line, even the 5 should be omitted and only the 89 written down.

Similarly, in expressing the grid location of a point, some of the digits of the coordinates are often

Figure 9-16.-Portion of a military map.

omitted; for example, the grid location of the bullion depository at Fort Knox may be given as 16SES90999315. This means zone-row 16S, 100,000- meter square ES, casting 9099, northing 9315. Actually, the casting is 590,990 and the northing 4,193,150.

If four digits are given in a coordinate element, the coordinates pin a point down to a particular 10-meter square. Consider figure 9-17, for example. For the point X, the two-digit coordinates 8893 would mean that the point is located somewhere within the 1,000- meter-grid square 8893. To pin the location down to a particular 100-meter square within that square, you would have to add another digit to each coordinate element. The X lies four-tenths of 1,000 meters between line 88 and line 89; therefore, the casting of the 100-meter square is 884. By the same reasoning, the northing is 933. The coordinate for the 100-meter square is therefore 884933. To pin the point down to a particular 10-meter square, you should add another pair of digits, these being determined by scale measurement on the map. It follows from all this that the coordinates previously given for the bullion at Fort Knox (909993 15) locate this building with reference to a particular lo-meter square.

Figures 9-18 and 9-19 show the marginal information usually given on a UTM grid military map. Note the reference box, which gives the grid zone-row and 100,000-meter-square designation. The

Figure 9-21.Appearance of meridians and parallels on a conic projection.