4-3. MILITARY GRIDS
An examination of the transverse Mercator projection, which is used for large-scale military maps, shows that most lines of latitude and longitude are curved lines. The quadrangles formed by the intersection of these curved parallels and meridians are of different sizes and shapes, complicating the location of points and the measurement of directions. 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 types of grids vary, but three properties are common to all military grid systems: one, they are true rectangular grids; two, they are superimposed on the geographic projection; and three, they permit linear and angular measurements.
a. Universal Transverse Mercator Grid. The UTM grid has been designed to cover that part of the world between latitude 84° N and latitude 80° S, and, as its name implies, is imposed on the transverse Mercator projection.
(1) Each of the 60 zones (6 degrees wide) into which the globe is divided for the grid has its own origin at the intersection of its central meridian and the equator (Figure 4-8). The grid is identical in all 60 zones. Base values (in meters) are assigned to the central meridian and the equator, and the grid lines are drawn at regular intervals parallel to these two base lines. With each grid line assigned a value denoting its distance from the origin, the problem of locating any point becomes progressively easier. Normally, it would seem logical to assign a value of zero to the two base lines and measure outward from them. This, however, would require either that directions— N, S, E, or W— be always given with distances, or that all points south of the equator or west of the central meridian have negative values.
Figure 4-8. UTM grid zone location
(2) This inconvenience is eliminated by assigning "false values" to the base lines, resulting in positive values for all points within each zone. Distances are always measured RIGHT and UP (east and north as the reader faces the map), and the assigned values are called "false easting" and "false northing. " (Figure 4-9). The false eating value for each central meridian is 500,000 meters, and the false northing value for the equator is 0 meters when measuring in the northern hemisphere and 10,000,000 meters when measuring in the southern hemisphere. The use of the UTM grid for point designation will be discussed in detail in paragraph 4-4.
Figure 4-9. False eastings and northings for the UPS grid.
b. Universal Polar Stereographic Grid. The UPS grid is used to represent the polar regions. (Figure 4-10)
Figure 4-10. Grid zone designation for UPS grid
(1) North Polar Area. The origin of the UPS grid applied to the north polar area is the north pole. The "north-south" base line is the line formed by the 0-degree and 180-degree meridians; the "east-west" base line is formed by the two 90-degree meridians.
(2) South Polar Area. The origin of the UPS grid in the south polar area is the south pole. The base lines are similar to those of the north polar area.