A typical note may read “To convert a magnetic azimuth to grid azimuth, subtract
G-M angle.” If you have a magnetic azimuth of 270 degrees, and the G-M angle is 8 degrees,
your grid azimuth will be 262 degrees.
The conversion (whether to add or subtract) depends on whether your map has an
easterly or westerly G-M angle. If your magnetic north is to the right (east) of the grid
north, then your map has an easterly G-M angle. If your magnetic north is to the left (west)
of the grid north, then your map has a westerly G-M angle.
You will learn more about azimuths and land navigation as you progress through
ROTC. By the end of your MSL III year, you must master land navigation in order to succeed
at the Leader Development and Assessment Course (LDAC), which you will attend at
Fort Lewis, Wash., after your MSL III year.
Determining Elevation
You can determine the elevation of any location on your map without any special equipment
using two things on your map that you learned about in the previous Map Reading
section—contour lines and the contour interval. Before you can determine the elevation
of any point on your map, you must first know the contour interval for the map you are
using. As you read previously, you can find the contour interval in the margin of your
map—usually in the middle of the lower margin. Recall that the contour interval is a
measurement of the vertical distance between adjacent contour lines.
Refer to Figure 5.7 to learn how to determine the specific elevation of a point on
a map:
1. Identify the contour interval and the unit of measure used (feet, meters, or
yards) from your map’s marginal information at Figure 5.6 (most military maps
use meters).
Using the map example at Figure 5.7, if you wanted to determine the elevation
to point A, you would find the numbered index contour line nearest point A. In
Figure 5.7, the closest numbered contour line to point A is the 500-meter
contour interval.
Determine if point A is above (higher in elevation) the 500-meter contour line,
or if point A is below (lower in elevation) than the 500-meter line. Since point A
lies between the 500-meter contour line and the 600-meter contour line, moving
from the closest contour line (500-meter) to point A would be traveling uphill to
a higher elevation.
2. Determine the elevation of point A by starting at the index contour line
numbered 500 and counting the number of intermediate contour lines (the
unmarked contour lines) to point A.
Point A is on the second intermediate contour line above the 500-meter index
contour line. Since the contour interval is 20 meters (Figure 5.6), each
intermediate contour line crossed to get to point A adds 20 meters to the 500-
meter index contour line. The elevation of point A is 540 meters.
3. Determine the elevation of point B by going to the nearest index contour line. In
this case, it is the upper index contour line, numbered 600. Point B is located on the
intermediate contour line immediately below the 600-meter index contour line.
Therefore, point B is located at an elevation of 580 meters. Remember, if you are
increasing elevation, add the contour interval to the nearest index contour line.
If you are decreasing elevation, subtract the contour interval from the nearest
index contour line.
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