Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 5.

SECTION V: DIRECTION AND AZIMUTH

15. GENERAL. Distance and direction are used to locate points or objects on the ground or on a map in relation to known points. The distance is measured, paced, or estimated, depending on the degree of accuracy required (see sec. IV). For military purposes, direction is always expressed as an angle from some fixed or easily established base direction line.

16. UNITS OF ANGULAR MEASUREMENT. a. General. The value of an angle is expressed in degrees (°), minutes ('), and seconds (''); or in mils (see fig. 7). Personnel in artillery or heavy weapons units use the mil since fire-control instruments are generally graduated in mils. Other personnel usually use degrees, minutes, and seconds.

b. Degrees, minutes, and seconds. When the circumference of a circle is divided into 360 equal parts by lines drawn from the center to the circumference, the angle between any 2 adjacent lines is 1°. Degrees are divided into minutes and seconds thus:

A circle =360° (degrees)

1° (degree) = 60' (minutes)

1' (minute) = 60" (seconds) Angles are written as 137° 45' 23''.

c. Mils. If the circumference of a circle is divided into 6,400 equal parts by lines from the center to the circumference, the angle between any 2 adjacent lines is 1 mil. Thus an angle would be written in mils, as 1,327 mils. The mil is useful to artillery and heavy weapons units because it is an angle the tangent of which is approximately 1/1,000. Therefore a change of 1 mil in the direction of a machine-gun barrel changes the center of impact of a bullet 1 yard at a range of 1,000 yards or 2 yards at 2,000 yards.

Figure 7. Angles are expressed in degrees or mils.

d. Relation 'between degrees and mils. Degrees are changed to mils or mils to degrees by the following conversion factors:

360° = 6,400 mils

                 17.8 mils (or 18 approximately)

        1 = 360 = 1

7

Hence 10° = 10 x 17.8 = 178 mils (or 180 approximately)

17.8 mils (or 180 approximately)

1 mil =.056°  (or 3.4' approximately)

Hence 100 mils = 100 x .056 = 5.6° or 5° 36'

17. BASE DIRECTION. Direction from one point to another is always expressed as an angle from a base line. There are three base directions, namely, true north, magnetic north, and grid north, usually shown on maps by a star, half arrowhead, and the letter "y," respectively (fig. 8). Grid north may also be designated by "GN."

Figure 8. Declination diagram.

a. True north. True north is the direction to the north pole. It is used in surveying where great accuracy is required but is not normally used by military personnel in the field. Meridian or longitude lines on a map represent true north and south directions.

b. Magnetic north. Magnetic north is the direction of the north magnetic pole. It is indicated by the N (northseeking) end of the compass needle. It is ordinarily used for field work because it can be found directly with the common compass.

c. Grid north. Grid north is the direction of the vertical grid lines usually found on military maps (see par. 27). Determination of direction by grid north is convenient because grid lines are located at frequent intervals on maps.

18. DECLINATION. a. General. Declination is the difference in direction between true north and magnetic north or between true north and grid north. There are therefore two declinations, magnetic and grid. In figure 8, magnetic declination is 6° 40' west and grid declination is 2° 25' east.

b. Magnetic. Magnetic declination is the angle between true north and magnetic north. In localities where a compass needle points east of true north, magnetic declination is east. Where a compass needle points west of true north, magnetic declination is west. Where true north and magnetic north are the same, magnetic declination is zero. Lines joining points on the surface of the earth where magnetic declination is zero are called agonic lines. Lines joining points having the same magnetic declination are called isogonic lines. The magnetic declination in the United States varies from 25° east in Washington State to 22° west in Maine. Isogonic lines run generally north and south in the United States but are irregular because of local conditions. The magnetic declination at any one locality is subject to a gradual change, the amount of which can be predicted from past records. This change in some localities in the United States is as much as 4° annually. For example, in figure 8 the annual increase is 3'. Hence, in the 9-year period 1935-1944, magnetic declination has increased 9 times 3' or 27', and the 1944 magnetic declination is 6° 40' plus 27', or 7° 07' west of true north. When the magnetic prong is plotted in this position, the G-M angle (par. 19b) is increased 27', from 9°' 05' to 9° 32'. Annual change is frequently expressed as E or W to avoid ambiguity in the change of direction.

c. Grid. Grid declination or gisement is the fixed difference in direction between true north and grid north. Grid declination varies in different localities. Actually, it varies at different points on any one map, but on a tactical map the variation is so slight that the average declination can be used without introducing an appreciable error. Figure 9 shows the reason for grid declination. The rectangle abcd is a mapped area or zone with the lines ef and gh shown as military grid lines. The dashed lines are the true north and south lines. The grid line JK is parallel to the central meridian, a true north-south line. Other grid lines in the zone are drawn parallel to JK. Hence a map made at position I has grid declination west, while a map made at position II has grid declination east. In the military grid system, grid declination ranges from 3° east to 3° west in the United States. Elsewhere in the world, when other grid systems are used, the maximum grid declination is much greater and depends upon the width of the zone. Military maps show in diagrammatic form the average grid declination for the area represented.

Figure 9. Diagram illustrating reason for grid declination.

19. DECLINATION DIAGRAMS. a. General. A declination diagram is printed on the margin of military maps. It has three prongs showing the directions of true, magnetic, and grid norths. The angles between prongs are usually drawn accurately and can be used for graphic work on the map. For reasons given below declination diagrams should be verified by measurement before being used for this purpose. On some maps when the declination is small the diagram is exaggerated. In these cases, and generally on maps printed since 1943 the following note appears under the diagram:

USE DIAGRAM ONLY TO OBTAIN NUMERICAL VALUES

Diagrams so marked will not be used for graphical purposes. See figure 8.

b. G-M angle. (1) The angle between grid north and magnetic north is commonly called the G-M angle. The angle is west when magnetic north is west of grid north; east when magnetic north is east of grid north. It is used frequently in field map reading and its exact value is given in degrees on the declination diagram of new maps. However, its value may not be listed on older maps. In such cases, the G-M angle is computed by adding magnetic and grid declinations when magnetic and grid prongs are on opposite sides of the true-north prong and by subtracting when they are on the same side. Once the G-M angle has been computed, it should be written on the map for easy reference.

(2) An increase in annual magnetic change may increase or decrease the G-M angle. If the magnetic prong moves toward the grid prong, the G-M angle is decreased; if it moves away, the G-M angle is increased.

c. Two-pronged diagrams. As explained in chapter 6, the world is divided into zones for map-making purposes; each zone has a military grid drawn parallel to the central meridian of the zone. Hence, the grid declinations of adjoining zones are different and any map of a border line area has two grid systems and two grid declinations (see fig. 10). In such cases, a two-pronged diagram showing the relation between true and magnetic north at the center of the sheet is shown in black in the margin. Adjacent to the diagram are notes giving the grid declination for the center of each gridded area. These notes are usually in the color of the grid to which they refer.

Figure 10. Declination diagram for map on which two zones appear.

20. AZIMUTH. In describing the position of one point on a map or in the field with reference to some other point, the army uses the azimuth system of measuring direction. Military azimuths are horizontal angles measured clockwise from magnetic-, true-, or grid-north base lines.

a. Magnetic azimuth. The magnetic azimuth of any line is the horizontal angle measured clockwise from magnetic north to the line. For example, in figure 11 the magnetic azimuth of the line from the road junction to the church is 60°.

b. Grid azimuth. The grid azimuth of any line is the horizontal angle measured clockwise from grid north to the line. In figure 11 the grid azimuth is 51°.

c. True azimuth. The true azimuth of any line is the horizontal angle measured clockwise from true north to the line. In figure 11 it is 54°.

d. Relation between magnetic and grid azimuth. In the field, magnetic azimuths are read directly from the compass. If the map is one with the protractor and pivot point (see FM 21-25), the magnetic north line may be drawn easily on the map, and that line used to plot compass reading. However, on older maps, a compass reading is usually converted to grid azimuth before it is plotted on the map. The difference between grid and magnetic azimuth is the G-M angle.

(1) When magnetic north is east of grid north: grid azimuth = magnetic azimuth plus G-M angle.

Figure 11. Three kinds of azimuth.

(2) When magnetic north is west of grid north: grid azimuth = magnetic azimuth minus G-M angle. For example, in figure 11 grid azimuth = 60° - 9° = 51°.

e. Back azimuth. Back azimuth is simply the azimuth of a line viewed backward. (See FM 21-25.) The back azimuth of a line is its forward azimuth plus 180°, of if this sum is greater than 360°, the back azimuth is the forward azimuth minus 180°. For example, if the forward azimuth of a line is 50°, the back azimuth is 50° + 180° = 230°. Or if the forward azimuth of a line is 310°, the back azimuth is 310° - 180° = 130°.

21. BEARING. Bearings are used to express directions by the service watch compass, many of which are still in use. The bearing of a line is its horizontal angle and direction with respect to either north or south direction

Figure 12.      Relationship between azimuths and bearings.

(2) Direction expressed in azimuths and bearings.
Figure 12. - Continued.

line and never exceeds 90°. Figure 12 (1) shows how bearings are measured and indicates the relationship between bearings and azimuths. If bearings are magnetic the azimuths likewise are magnetic. Figure 12 (2) illustrates how to express a typical direction in each quadrant both as an azimuth and as a bearing.

22. PROTRACTOR. A protractor is an anstrument for measuring or laying off angles on a map. Figure 13 (1) and (2) illustrate two types; the semicircular type is the more common. The protractor represents half an azimuth circle but is graduated in two scales to represent a complete circle, one scale reading from 0° to 180°

(1) Semicircular protractor.
(2) Rectangular protractor.
Figure 13.

and the other from 180° to 360°. When the semicircular scale is placed as shown at the left in figure 14, only the scale 0° to 180° can be read. If the protractor is turned so the circular portion is to the left, the scale 180° to 360° can be read.

23. TO MEASURE AZIMUTH OF ANY LINE ON A MAP. Following are examples illustrating methods of finding azimuths on a map:

a. Problem 1. To find the grid azimuth of the line from the crossroads at A to the house at B in figure 14.

Extend the line AB until it intersects the 349 grid line. Lay a protractor on the map with its, index at this intersection and the straight portion lying along the 349 grid line. Read the grid azimuth of AB. It is 138°.

b. Problem 2. To find the magnetic azimuth of the line from the crossroads at C to house at D. Extend the line CD beyond the edge of the protractor. Lay the protractor on the map with its index at the intersection

Figure 14. Schematic illustration showing use of protractor to measure azimuth on map.

of CD with the 351 grid line and the straight portion lying along the 351 grid line. Since the azimuth of the line is greater than 180°, the scale reading from 180° to 360° is used. The grid azimuth of CD is 226°. The declination diagram shows the G-M angle to be 4° west, so it will have to be added to the grid azimuth. The magnetic azimuth is 226° + 4° = 230°. If the magnetic north line is plotted on the map with the aid of the map pivot point and partial protractor scale, the magnetic azimuth may be measured directly from this line.

24. TO PLOT AN AZIMUTH ON A MAP. a. Grid azimuth. Problem: To plot from CR 685 on figure 15 a line with a grid azimuth of 75°. Construct a line

Figure15. Schematic illustration showing use of protractor to plot a given azimuth on a map.

through the crossroads parallel to the north-south grid. Place a protractor on the map with its base on the line and its index at the crossroads. Plot the point P at the 75° reading on the protractor. Remove the protractor and draw a line from the crossroads through P.

b. Magnetic azimuth. To plot the magnetic azimuth of a line, follow the same procedure as in a above but construct the line through the crossroads parallel to magnetic north, rather than to the north-south grid or convert magnetic azimuth to grid azimuth and plot as directed in a above.

25. COMPASSES. FM 21-25 explains how to use a compass. In addition to variation caused by magnetic declination, a magnetic compass is affected by the presence of iron, magnets, and charged electric wires and electric apparatus. Certain geographic areas have deposits of mineralized rock (such as iron ore) which render a compass unreliable in those vicinities.

Figure 16. Watch compass gives direction in bearings.

Consequently, all visible masses of iron or electrical fields must be avoided when using the compass. The following are the minimum safe distances:

The four common types of compass are the watch, lensatic, prismatic, and wrist.

a. Watch compass. The face of the watch compass (fig. 16) is divided into quarters, or quadrants, of 90° each. Readings are given as bearings as explained in paragraph 21. This compass has no sights.

b. Lensatic compass. The standard compass for general use in our Army is the liquid-filled lensatic, so called because azimuths are read through a magnifying lens in the eye piece. Figure 17 shows the lensatic compass and its nomenclature. FM 21-25 explains how to use it. The newest lensatic compass (fig. 18) differs from the model in figure 17 in that the floating dial is transparent and is graduated in mils as well as in degrees. Numbers on the dial are printed in black. There is a fixed luminous sector on the inside of the case which permits reading azimuths at night.

c. Prismatic compass. The prismatic compass shown in figure 19 differs from the lensatic compass in that azimuths are read from the dial through a prism rather than through a lens.

(1) The compass consists of a case housing a magnetic dial, a hinged cover with a glass window, and an eyepiece containing a prism for reading graduations on

Figure 17. Nomenclature of old issue lensatic compass.

the dial. The dial has two scales, the outer one to be read through the prism of the eye piece, the inner one to be read directly at the front sight. Both are graduated from 0° to 360°. The north point is indicated by a luminous arrow.

(2) The glass cover has an etched line which is used like the hair line on the lensatic. Closing the cover operates a lever which raises the dial off the pivot to protect it. To release the dial the lever at the side must be pushed forward.

Figure 18. New issue lensatic compass.

(3) When the cover of the compass is raised, a glass disk protects the dial. The luminous index line used in setting azimuths at night is painted on this disk. The index line can be set at any desired angle simply by loosening the setscrew on the side and revolving the corrugated brass ring which houses the glass disk.

(4) The outside of the compass case is graduated in degrees, counterclockwise. For night use the luminous index line is set opposite the desired azimuth indicated on the outside of the case. The compass is rotated until the luminous arrow on the dial points to the luminous index line. Now the two luminous markers on the hinged

Figure 19. Nomenclature of prismatic compass.

cover point along the desired azimuth or direction of march.

d. Wrist compass. The wrist compass is a liquid filled compass designed to be worn strapped to the wrist as shown in figure 20. FM 21-25 explains how it is used.

Figure 20. Wrist compass.

26. TRAVERSE. A traverse is a series of connected lines of known distance and direction. A traverse is useful in exploring unfamiliar terrain and in recording the course taken. To make one, start from a known point and follow observed compass courses from point to point, measuring distances. When plotted to scale on the map, these course lines and distances show graphically the course followed and the location of any desired point on the traverse. A typical traverse is shown in figure 21.

Figure 2l. A traverse is a series of connecting lines of known distance and direction.