Figure 5-4.--Grid system.
Figure 5-3.--Position location.
measurements. Military grids consist of two sets of
equally spaced parallel, straight lines intersecting at
right angles and forming a series of squares. Each grid
east-west rings running parallel to the equator. (See figs.
line is an even interval of the selected measurement unit,
5-2 and 5-3.) Parallels are used to measure
such as yards or meters. A portion of a military grid, or
LATITUDEthe distance of a point north or south of the
map, is shown in figure 5-1. The dimensions and
equator. Using meridians and parallels, you can locate
orientation of different types of grids vary, but all
any point on the surface of the earth.
military grids have three things in common:
Geographic coordinates are expressed in angular
1. They are all true rectangular grids.
measurement. The earth is divided into 360 degrees;
2. They are superimposed; that is, drawn on top of
each degree into 60 minutes; and each minute into 60
the geographic projection.
seconds. The degree is symbolized by ; the minute by;
and the second by.
3. They permit linear and angular measurements.
Starting with 0 at the equator, the parallels of
The regularly spaced lines that make up the grids on
latitude are numbered to 90 both north and south. The
any large-scale map are divisions of the 100 000-meters
extremities are the North Pole at 90 north latitude and
square; the lines are spaced at 10 000- or 1000-meter
the South Pole at 90 south latitude. Latitude can have
intervals. Each of these lines is labeled at both ends with
the same numerical value north or south of the equator,
a number showing its relation to the origin of the zone.
so the direction N or S must always be given.
For the 1000-meter grid, except for the numbers labeling
the first grid line in each direction from the southwest
Starting with 0 at the prime meridian, longitude is
corner of the sheet, the last three digits (000) of the
measured both east and west around the world. Lines
east of the prime meridian are numbered from 0 to 180
number are omitted. (See fig. 5-1.) Two digits of the
and are identified as east longitude. Lines west of the
numbers are printed in large type, and the same two
prime meridian are numbered 0 to 180 and are
digits appear at intervals along the grid line on the face
identified as west longitude. The direction E or W must
of the map. They are called the PRINCIPAL DIGITS
always be given to longitude. The line directly opposite
and represent the 10,000 and 1,000 digits of the grid
the prime meridian, 180, may be referred to as either
number; they are of major importance to the map reader
east or west longitude.
because they are numbers he uses most often for
referencing points. The smaller digits complete the
Grids
COORDINATES of the grid lines, but they are rarely
used for point designation. On sheets with grid line
Military maps are divided into grids to provide a
spacing at 10000 meters, only one principal digit is
uniform system for referencing and making
shown, representing the 10,000 digit of the grid number.
5-8
