MAP READING

CONTENTS CHAPTER VI

MAP READING ON THE FIELD

42. General.

The map and aerial photograph are often the only means available for studying distant or inaccessible terrain. Even when the terrain is accessible, the map, is still most valuable as a source of names, and a convenient means by which to find one's way about. The commander should always take his map with him into the field, and refer to it constantly. When operating over unfamiliar territory he should keep his movements plotted on his map, verify his location at every opportunity, and from his map learn the names of the terrain features encountered.

43. Terrain Feature Terms.

The standard terrain terms, such as hill, ridge, stream, crest, and the like, are used in map work and in the field. For use in the field to identify more readily the special and the minor terrain features, a large number of less common terms are used. Those most frequently encountered are shown in Plate 33.

44. Distance.

The determination of distance in the field presents many difficulties. Distance can be measured by pacing or by tape, but tbis method is slow, tedious, and often impracticable. Long road distances can be measured by the odometer of an automobile. Visible areas can be measured by estimation by eye. This requires a certain amount of skill and experience, and is not successful in the dark, in woods, over long distances, or in broken country. Two other methods of handling distance in the field are the landmark method, and the travel-time method. For example, a patrol is directed to proceed two miles down a road and take up a position in observation. The patrol leader could take a map, scale off two miles, study the map and select some recognizable feature in that vicinity. He would then march until he reached the feature. On the other hand, if no map were available, he might calculate that the two miles would require forty minutes of marching. He would march forty minutes and then take position.

45. Magnetic Compass.

a. Types.
The three types of compass issued. to the service are watch, prismatic and lensatic. The compasses are affected by presence of iron, steel, or electricity, and will not give accurate readings near an automobile, tank, field piece, machine gun, or power line. A steel helmet, rifle, or pistol on the person of the observer may influence the needle and make readings inaccurate.


PLATE 33 - Military Features of the Terrain.

b. Watch compass.
The watch compass has a movable needle, a fixed dial graduated in bearings, and has no sighting device. (Plate 34.) It is now being replaced by the lensatic compass.


PLATE 34 - Watch Compass.


PLATE 35 - Types of Military Compasses.

c. Prismatic.
The prismatic compass has a circular magnetic floating dial, prismatic eye piece, and a north point painted in luminous paint in the case. One of the two dial graduations is read through the prism and the other is read direct. Each dial is graduated from O degrees to 360 degrees. The forward sight is a hair line on the glass in top of the case. (Plate 36.)


PLATE 36 - The Prismatic Compass Open.

d. Lensatic compass.
The lensatic compass functions in much the same manner as the prismatic compass. It contains a magnifying lens fixed in a hinged eye piece. The face has two scales, one graduated in degrees, the other in mils (Plate 37).


PLATE 37 - The Lensatic Compass.

46. Measuring Azimuth With the Compass.

To read the azimuth of a point on the terrain (distant hill, house, etc.) proceed as follows:

Raise be cover and the eyepiece and unlock the dial. Hold the compass to the eye and sight the compass at the object. Hold the compass steady until the dial comes to rest. Read the azimuth figure on the dial at the index point. This will be the magnetic azimuth of the line from the observer to the object. See Plate 38. To establish a given azimuth on the ground, proceed as follows: Raise the cover and lens and unlock the dial. Permit the dial to come to rest. Hold the compass to the eye and watch the dial, facing the entire body about until the desired azimuth figure on the dial is at the index point. Holding the com-


PLATE 38 - Using the Compass in the Field.

pass in this position, look through the sights of the compass and pick up some ground feature on the line of sight. The line determined by this feature will be the azimuth desired.

47. Marching by Compass.

a. By day.
Orders to troops may direct them to attack cross-country in a pJescribed direction given in term,s of azimuth. Groups or individuals selecting cross-country routes from the map may-compute the azimuth of various legs of


PLATE 39 - Compass Set for Night Marching.

the trip to prevent the possibility of getting lost. In any such case, map azimuths must be converted to magnetic azimuths before they can be used with the compass. To march by compass, the commander rotates the compass until the dial reads the required azimuth. He then sights along the axis of the compass and selects some hill, house, tree, or other feature on this line (Plate 38). He then marches toward the


PLATE 40 - Orienting the Map by Compass.

above feature until he reaches it, or it becomes invisible. He then repeats the operation, selecting a new feature on the line of the required azimuth upon which to guide the march. This is continued until the goal is reached (the compass is not in use while actually marching). The compass is used to select successive features on the required line, and the actual marching is always conducted toward such visible feature. The more distant and prominent the feature the easier the procedure.

b. By night.
For use in marching at night, the compass is equipped with a movable luminous marker on the top of the case, and with a night-marching azimuth scale on the outside of the case near the base. To set the compass for night marching on a predetermined azimuth, rotate the ring on the top of the case until the luminous mark is at the proper azimuth on the night-marching scale. Hold the compass in the hand with the dial free and turn. the entire body until the luminous arrow of the dial points to the luminous marker. The direction of march is now the line of the axis of the compass (Plate 39). Select some features such as a low star, skyline hilltop or saddle, or other recognizable feature on this line and march in the direction of the feature selected. Repeat the operation as often as necessary, and make frequent checks of the direction while en route. The setting of the night-marching mark on its correct azimuth must be done before going into the field, or by flashlight screened from enemy ground and aerial observation by an overcoat, shelter half, or other means.

48. Orientation in the Field.

Whenever the map is studied in the field, it should be oriented to the ground. A map is "oriented" when the directions on the map are parallel to the corresponding directions on the ground. If any one direction on the map can be made parallel to its corresponding ground direction, all other directions will automatically become parallel to their corresponding ground directions also, and the map will be oriented. An individual in the field is said to be "oriented" when he knows his location, both in the field and on the map, and also knows the cardinal directions on the ground.

49. Orienting the Map in the Field.

a. By compass.
In unfamiliar territory, when the individual is not sure of his location, and when accuracy of orientation is desired, the fastest and best method of orientation is by compass. Place the map on a level surface. Place the compass, opened and with the dial free, upon the map so that the axis of the compass (etched line on the glass cover) is accurately superimposed on and in coincidence with a vertical grid line of the map. The cover of the compass must be toward the top of the map. Revolve the map and compass together until the needle is in the same relation to the grid line as is shown for the magnetic north arrow in the orientation symbol on the map. (See Plate 40).

b. By inspection.
When traveling by road, and when the reader knows his approximate location on the map, the map may be oriented as follows: Lay the map in the road. Rotate the map until the road as shown on the map is pointing down (is parallel to) the road itself. Any trail, stream, ridge, or other line identifiable on both the map and the ground can be used in the same manner.

c. By ground feature.
An individual in the field who knows his location, but who may not have a compass available, and who does not know the cardinal ground directions, can orient the map as follows: Study the ground and the map and select some distant feature recognizable both on the map and on the ground. On the map draw a line from the known map position to the above feature. Revolve the map until the line so drawn points toward the feature itself. The map will then be oriented. Sighting is facilitated by laying an alidade, straight edge, or pencil on the line.

50. Locating Oneself on the Map.

a. By inspection.
When an individual knows his approximate location on the map, he studies the visible terrain for distinctive features, and the map to locate and identify these features. He estimates the distance and direction to the features on the ground and notes the corresponding distances and directions on the map. Location by inspection is gFeatly simplified if the map is oriented to the ground.

b. By single point.
To locate one's position on the map while traveling a known road, proceed as follows: Orient the map. Select some distant feature of the terrain that can be located and identified on the map. Place a pin through the feature on the map.


PLATE 41 - Orienting the Map by Inspection.


PLATE 42 - Orienting the Map by Ground Feature.




PLATE 43 - Locating Position on the Map by Inspection.


PLATE 44 - Locating Position on the Map by Single Point.

Take an alidade, pencil, or any straight edge, hold it against the pin and tum it until it points at the feature on the ground. Draw a line on the map along the edge from the pin forward the road. The point where this line intersects the road is the location of the position. Check the results by studying the near-by terrain features and comparing them with the map.

c. By resection.
(See paragraph 29.) To locate one's position on a map in the field by resection, proceed as follows: Orient the map accurately. Select a distant visible feature of the ground, and locate and identify it on the map. Place a pin in the feature, place an alidade or any straight edge against the pin, and turn it until it points at the feature on the ground. Draw a ray on the map from the pin toward your position. Select a second feature, at as nearly a right angle as possible from the first feature. Repeat the operation. The intersection of the two lines is the desired map location. During the entire procedure the map must remain oriented.


PLATE 45 - Locating Position on the Map by Resection.

51. Identifying Features in the Field.

Features, such as hills and woods, are shown on the map in their horizontal plan. In the field, one sees these features in profile instead of in plan, and their characteristic sizes and shapes may not be apparent. An officer directed to proceed to "SMITH HILL" may see several hills to his front, all of similar appearance, and be in doubt as to which one is "SMITH HILL." Or again, an officer in the field in unfamiliar territory, may wish to report some activity noticed in one of several clumps of woods of indefinite and indistinguishable identity. The map is a source of names and therefore of identification. The use of the map for this purpose is one of its most important military uses in the field.

a. To identify on the ground a feature shown on the map.
Orient the map. Locate own position on tbe map and place a pin in this position. Locate the feature on the map and place another pin in its position. Lay a straight-edge against both pins and sight along the straight-edge. The ground feature will lie on this line of sight, and at the distance indicated by the map.

b. To locate or to identify on the map a feature seen on the ground.
Single ray method. Orient the map. Locate own position on the map and place a pin in this position. Lay a straight-edge against the pin and sight the straight-edge at the ground feature, keeping the straight-edge in contact with the position pin. Draw a ray (line) on the map on the pin side of the straight-edge. Estimate the ground distance to tbe feature, and scale this distance off along the ray. This point will be die map location of the feature.

c. To locate on the map a feature seen on the ground. Intersection method.
Orient the map. Locate own position thereon, and place a pin in this position. Lay a straight-edge against the pin and sight the straight-edge at the ground feature. Draw a line on the pin side of the straight-edge. Proceed to some other location from which the feature is visible, and repeal the operation from this point. The intersection of the two lines will be the map location of the feature. (See paragraph 29.)