Military Instructors Manual Part 22

2. A Statement in words or figures, e.g., 3 inches equal one mile, meaning that 3 inches measured anywhere on the map represent 1 mile on the actual ground.

3. The Representative Fraction (generally known abbreviated as R.F.) having a number above the line that shows the unit length on the map and below the line the number of units which are in the corresponding actual ground distance. For example, if 1" = 1 mile, then the R.F. is:

1" (map distance) ----------------------------------- 63,360" (1 mile--ground distance)

if 3" = 1 mile the R.F. is: 3" (map distance) 1 ----------------------------------- or ------- 63,360" (1 mile--ground distance) 21120

if 6" = 1 mile: 6" 1 -------- or ------- 63360" 10560

if 12" = 1 mile: 12" 1 --------- or ------ 63360" 5280

In reading a map one must know the scale and also where the North is.

This is always indicated by an arrow pointing either to the magnetic North or the true North. If to the magnetic North the needle will have but one barb away from the true North. The angle between the magnetic and the true North is the declination.

Placing the map in proper relation to the ground so that points of the compa.s.s coincide on map and ground is called _orienting the map_.

In map work there is one vital point to remember; practically all the ground surface is in its present form as a result of water action

1. Look for the water courses, that is the drainage system. It will give the general slope of the land.

2. Look for the high points between the water courses, remembering that there is always a valley then a hill then a valley again continued in succession.

3. Finally locate towns, railroads, main highways and work down to the minor details.

In measuring a map to get the actual distances on the ground, copy the graphic scale on any piece of paper and apply this directly or if your distances exceed your scale use the edge of a piece of paper and then apply it to the graphical scale on the map.

LESSON 2. (FIELD WORK--FIVE HOURS.)

_Problem--Stride Scale Map Making_.

Producing a map from the actual ground requires certain instruments.

The second lesson takes up the preparation of the stride scale on the alidade and the different kinds of maps, made in military sketching.

The alidade is a triangular ruler with one or more working scales on it beside other measurements. The _working scale_ is, for infantry, the stride or the s.p.a.ce of ground covered from left foot to left foot again in walking, reduced to the proper map distance. This varies with individuals of course. Any scale of units, however, can be used as, horse trot, telegraph poles, etc.

The working scale for each man is made by having him step off a measured course, say 440 yards. The ground should not be too even as a general average is needed, moreover the pace must be the natural gait of the individual under ordinary circ.u.mstances. Let him count the course three times then average the three results for the final estimate.

Now to convert this into a working scale for the alidade made on a scale of six inches to the mile; take the case of a man who takes 220 strides in 440 yards:

440 yards = 15,840 inches 15,840 220 = 72, or his stride in inches then 880 strides = 1 mile or 6" on the scale.

It is better to have a scale of 1,000 strides which is easily done by the proportion:

1,000 sts. : 880 sts. : : x : 6 x = 6.8

now draw a line 6.8 inches long and a diagonal line from it; divide this diagonal line into 10 equal parts for each 100 paces at any convenient scale and draw a line from the end of the tenth part to the end of the 6.8 inches line; draw lines parallel to this line from each of the divisions. The 6.8" line is then divided into 10 equal parts; each of these parts may be divided in the same manner into tenths.

Your scale is ready to be pasted or transferred to the alidade and each 6.8. inches on the map will equal 1,000 of your strides on the ground, or about 1-1/12 miles (2,000 yards).

There are two general cla.s.ses of sketches:

1. ROAD SKETCH.--A traverse (pa.s.sing over) made along a definite rout showing all features of military importance for a distance of 200 or 300 yards on each side of the road. A road sketch is always made on a scale of 3 inches to 1 mile.

2. AREA SKETCH.--A map of a definite locality. There are 3 kinds of area sketches according to opportunity for observation:

(a) Position Sketch--when access may be had to the whole area.

(b) Outpost Sketch--where part of the ground must be mapped without pa.s.sing over it. This form is applicable particularly to trench warfare. Intersection and resection are used to locate points within the enemy's lines.

(c) Place Sketch--when sketch must be made from one point, as when the proximity of the enemy would prevent any movement; as from trench observation stations, etc.; also an elaboration of the _landscape_ or _horizon_ sketch which is used everywhere in the trenches today. From one point an actual outline of the opposite trench and background is made in perspective, reference points on the horizon being marked on the edge of a pad at arm's length. These marks are then prolonged on the paper and the horizon is sketched. In like manner the middle distance and the foreground come under observation and are put on in one below the other.

Time must be allowed the men to make their stride scales and to paste or transfer them to their alidades.

[Ill.u.s.tration: Plate #9]

LESSON 3. (FIELD WORK--FIVE HOURS.)

The problem is to make a Position Sketch about one mile square closing the traverse. First considering the sketch board, compa.s.s, pencils, etc.; next the orienting, sighting and pacing; finally the uses of intersection and resection and in making allowance for error.

The sketch board should be about 12 to 18 inches square, being used with or without a tripod. A cheap camera tripod is excellent. The board should have a compa.s.s attached so that it will remain in the same relative position on the board. If iron thumb tacks are used avoid getting them too near the compa.s.s. A hard pencil must be used to obtain good results. The paper must be smooth and where possible covered with another sheet fastened on but one side which will readily fold back when one desires to work on the sketch.

By always placing the board so that the compa.s.s reads North it will be oriented correctly. Care must be used when near electric wires or ma.s.ses of metal as automobiles, railroad tracks, etc., which will attract the needle from its true azimuth (N. and S. direction) and thus throw off the whole map. In such cases it is far better to back sight and use the compa.s.s only at intervals to verify the sights.

This brings up the matter of sighting. It is important to make long shots thus reducing the amount of individual error. In taking a sighting point make sure it can be recognized when reached and make sure to look at the reverse side in order to recognize it in case of back sighting if necessary. Always carry several large-headed pins using one at your present station and resting the side of the alidade against it, swinging the other end for sighting.

After sighting and lining the sight on your sketch, step off evenly to pace the distance. Time is always a factor in military mapping and where possible make mental notes as you go along as to where roads or other important features are located, so that you can place them in their proper place on the map when you have reached the next station.

It is well always to set a good pace for here time can be readily saved.

Making an _intersection_ is very simple. For as the sketcher moves along he ties his map together by sighting at any prominent object near his area, running these lines very lightly and only where he a.s.sumes the points to lie on his map. An abbreviation on the line or a number referring to a list off to one side will answer to recall the object. At any other station where the same point can be seen a similar line is drawn and where the two lines cross will be the location of the object. In the case of three lines not crossing at the same point take the middle of the triangle so formed.

_Resection_ is just the reverse of this process. The mapper wants to know where he is located on the map. If he is properly oriented and can aim at two points on the ground which he has located on the map, he places a pin at one of these locations on the map and aims with the alidade at the object on the ground drawing a line towards himself; this is repeated with the other known point and where the two lines cross on the map will be the point he is standing at.

In intersection the greatest accuracy is obtained by running the rays so as to meet as nearly as possible at right angles.

In running a traverse the sketcher must expect to find some error at his closing point. This error must be distributed over the whole traverse so as not to have all the error concentrated at one point.

LESSON 4. (FIELD WORK--FOUR HOURS.)

PROBLEM.--Make a simple sketch, containing topographical details using the traverse made during the preceding lesson. Use of conventional signs should be emphasized and the appreciation of features of military importance impressed. A tendency is to put in details to a point of confusion. Judgment must be developed to choose telling points.

A sharp pencil is always needed in sketching; in putting in the topographical details special attention must be given to the pencil.

Keep the point sharp and make clear, distinct signs.

LESSON 5. (CLa.s.s ROOM--FOUR HOURS.)

PROBLEM.--Contours, the Vertical Interval, Use of the Slope Board, Map Distance, Visibility and Profiles.

A contour is an imaginary line on the surface of the earth all points of which have the same elevation from a base or datum level, sea level usually being this base. Slice an apple into pieces 1/2-inch thick; where the cuts come may represent the contour lines. Take these individual slices, beginning at the bottom and outline them on a sheet of paper with a pencil (having run a nail through the apple first to keep each piece in place). The resulting circles will represent the apple's outline at 1/2-inch intervals.