Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 8.

SECTION VIII: ORIENTATION

50. GENERAL. FM 21-25 covers the most frequently used methods of orienting a map. In addition, this chapter gives further data on orientation and on location of points by resection and intersection.

51. EXPEDIENT METHODS OF DETERMINING TRUE NORTH. a. Finding compass declination (fig. 55). To find the declination of a compass, take a magnetic azimuth of the sun, a planet, or a bright star at its rising and setting on one day or at its setting one day and its rising the next. Add these two azimuths and take one-half the difference between their sum and 360°. If the sum is less than 360°, the declination of your compass is east; if the sum is greater than 360°, the declination is west. For example, in figure 55:

Figure 55. Determining north by rising and setting of sun

Magnetic azimuth of rising sun                                    110°

Magnetic azimuth of setting sun                                  270°

The sum                                                                           380°

The difference between this sum (380°) and 360° is 20°

Take half of this difference, which is                        10°

Magnetic declination is 10° and it is a west declination because the sum of the azimuths is greater than 360°.

Compass readings should be taken when the celestial body is just above the horizon or at a gradient of zero. If this cannot be done, obtain both the rising and setting reading at the same gradient. The gradient may be determined by a clinometer. When a star is used, choose one which rises nearly east of the point of observation. Points from which observations are taken must be within 10 miles of each other.

b. Sun and plumb line method. In the north temperate zone, true north can be found by the following method. On level ground, lean a pole in the general direction of north as shown in figure 56. Suspend a weight from the end of the pole so it nearly touches the ground. Drive a peg a in the ground under the weight. About one hour before noon, attach a piece of string to the peg, and attach a sharpened stick to the other end of the string. The length of the string ab is the distance from the peg a under the weight to the shadow of the tip of the pole b. Drive a peg under the shadow of the tip of the pole at b; with the sharpened stick, draw an arc bcd on the ground. About 1 hour after noon the shadow of the tip of the pole will again cross the arc at d. Drive a peg at this point and tie a string between the two pegs bd on the arc. Drive a peg at e halfway between b and d or divide the arc bcd

Figure 56. Determining north by sun and plumb line.

into two equal parts at f. For practical purposes, true north is in the direction of the line aef. In the south temperate zone the same procedure is used but the line aef points south.

52. LOCATION OF DISTANT POINT BY INTERSECTION.

a. With compass and protractor. Distant or inaccessible objects can be located on a map by intersecting lines from two points of known position. For example, to find the enemy gun position shown in figure 57 (1), compass readings are taken from CR 685 and RJ 573. The readings of 46° and 333° are converted to grid azimuth and plotted on the map with a protractor. The intersection of the plotted lines marks the location of the enemy's piece, as shown in figure 57 (2).

b. Graphic method. The gun can also be located graphically, as shown in figure 58. AT RJ 573, the observer orients his map by compass or by inspection,

Figure 57. Distant point located on map by intersection compass and protractor.

levels it, and places a pin in the map position of RJ 573. He places a straightedge against the pin, sights along it to the enemy position, and then draws a line on the map through the position. He moves to CR 685, orients and levels the map, and draws a similar line

Figure 58. Distant point located on map by interjection (graphic method)

from CR 685. The intersection of the two lines locates the enemy position on the map.

53. RESECTION. Resection is a method to locate one's own position on the map.

a. Resection with compass. In figure 59 (1) the problem is for the men in the forward OP to locate themselves on the map in figure 59 (2). Compass readings are taken from the OP to RJ 171 (323°) and to CR 162 (34°) which can be identified and located on the map. To plot this on the map the compass readings must be converted to back azimuth and then to grid azimuth. The forward magnetic azimuth of line "OPRJ 171" is 332°, and the back magnetic azimuth is 332° minus 180°, or 152°. To find the grid azimuth the G-M angle 9° is subtracted; the answer is 143° grid azimuth. This procedure is repeated for line "OPCR 162." The intersection of these lines is the location of the observation post. Location'of the position is determined by reading two angles and plotting two lines.

b. Resection with map. Another method is to stick pins in the map positions of RJ 171 and CR 162 (fig. 60).

Figure 59. Position located on map by resection from two distant using compass and protractor.

Orient and level the map, lay a straightedge against a pin at CR 162, sight along it at the crossroads, and draw a line on the map. Without moving the map, repeat the operation with the pin at RJ 171. Your position is where the lines intersect.

Figure 60. Position located on map by resection from two distant points (graphic method).

°c. Tracing-paper method. Tracing paper can also be used in resection. First, choose three distant objects represented on the map, for example, a church, the corner of a fence, a road junction (fig. 61). These points should be so located that lines to them from the occupied position make angles not less than 30' nor more than 150°. Stick a pin in the tracing paper at

Figure 61. Position located on map by resection from three distant points (tracing-paper method).

any convenient assumed position of the observer, place a straightedge against the pin, sight at each of the three objects in turn without moving the tracing paper, and draw a line toward each object from the pin. Lay the tracing paper on your map and move it around until each line passes through the conventional sight representing the object toward which it was sighted. Now, the pinhole in the paper is directly over your position on the map. The three-point resection should, if possible, be made on points which form a triangle containing the position sought.