Manual of Military Training Part 150

MILITARY SKETCHING

(While this chapter presents the princ.i.p.al features of military sketching in a simple, clear manner, attention is invited to the fact that the only way that any one who has never done any sketching can follow properly the statements made, is to do so with the instruments and the sketching material mentioned at hand.

In fact, the only way to learn how to sketch is to _sketch_.)

=1878.= A military sketch is a rough map showing the features of the ground that are of military value.

Military sketching is the art of making such a military sketch.

Military sketches are of three kinds:

Position sketches, Fig. 1;

Outpost sketches;

Road sketches.

All kinds of military sketches are intended to give a military commander detailed information of the ground to be operated over, when this is not given by the existing maps, or when there are no maps of the area.

The general methods of sketching are:

(1) The location of points by intersection.

(2) The location of points by resection.

=1879. Location of points by intersection.= To locate a point by intersection proceed as follows: Set up, level and orient the sketching board (Par. 1872), at A, Fig. 1. The board is said to be oriented when the needle is parallel to the sides of the compa.s.s trough of the drawing board, Fig 2. (At every station the needle must have this position, so that every line on the sketch will be parallel to the corresponding line or direction on the ground.) a.s.sume a point (A) on the paper, Fig. 1 Y, in such a position that the ground to be sketched will fall on the sheet. Lay the ruler on the board and point it to the desired point (C), all the while keeping the edge of the ruler on the point (A), Fig. 1 Y. Draw an indefinite line along the edge. Now move to (B), Fig. 1 X, plotted on the map in (b), Fig. 1 X, and having set up, leveled and oriented as at (A), Fig. 1 Y, sight toward (C) as before. The intersection (crossing) of the two lines locates (C) on the sketch at (c), Fig. 1 X.

[Ill.u.s.tration: Fig. 1]

=1880. Locating points by resection.= A sketcher at an unknown point may locate himself from two visible known points by setting up and orienting his sketching board. He then places his alidade (ruler) so that it points at one of the known points, keeping the edge of the alidade touching the corresponding point on the sketch. He then draws a ray (line) from the point toward his eye. He repeats the performance with the other visible known point and its location on the map. The point where the rays intersect is his location. This method is called _resection_. However, local attractions for the compa.s.s greatly affect this method.

=1881. The location of points by traversing.= To locate a point by traversing is done as follows: With the board set up, leveled and oriented at A, Fig. 1 Y, as above, draw a line in the direction of the desired point B, Fig. 1 X, and then move to B, counting strides, keeping record of them with a tally register, Fig. 3, if one is available. Set up the board at B, Fig. 1 X, and orient it by laying the ruler along the line (a)-(b), Fig. 1 X, and moving the board until the ruler is directed toward A, Fig. 1 Y, on the ground; or else orient by the needle as at A. With the scale of the sketcher's strides on the ruler, lay off the number of strides found from A, Fig. 1 Y, to B, Fig. 1 X, and mark the point (b), Fig. 1 X. Other points, such as C, D, etc., would be located in the same way.

=1882. The determination of the heights of hills, shapes of the ground, etc., by contours.= To draw in contours on a sketch, the following steps are necessary:

[Ill.u.s.tration: Fig. 2]

(a) From the known or a.s.sumed elevation of a located station as A, Fig. 1 Y, (elevation 890), the elevations of all hill tops, stream junctures, stream sources, etc, are determined.

(b) Having found the elevations of these critical points the contours are put in by s.p.a.cing them so as to show the slope of the ground along each line such as (a)-(b), (a)-(c), etc., Fig. 1 Y, as these slopes actually are on the ground.

[Ill.u.s.tration: (Tally Register)--Fig. 3]

[Ill.u.s.tration: (Clinometer)--Fig. 4]

To find the elevation of any point, say C (shown on sketch as c), proceed as follows:

Read the vertical angle with slope board, Fig. 2, or with a clinometer, Fig. 4. Suppose this is found to be 2 degrees; lay the scale of M. D.[22] (ruler, Fig. 2) along (a)-(c), Fig. 1 Y, and note the number of divisions of -2 degrees (minus 2) between (a) and (c).

Suppose there are found to be 5-1/2 divisions; then, since each division is 10 feet, the total height of A above C is 55 feet (5-1/2 10). C is therefore 835 ft. elev. which is written at (c), Fig. 1 Y.

Now looking at the ground along A-C, suppose you find it to be a very decided concave (hollowed out) slope, nearly flat at the bottom and steep at the top. There are to be placed in this s.p.a.ce (a)-(c), Fig. 1 Y, contours 890, 880, 870, 860 and 850, and they would be s.p.a.ced close at the top and far apart near (c), Fig. 1 Y, to give a true idea of the slope.

The above is the entire principle of contouring in making sketches and if thoroughly learned by careful repet.i.tion under different conditions, will enable the student to soon be able to carry the contours with the horizontal locations.

=1883.= In all maps that are to be contoured some plane, called the _datum plane_, must be used to which all contours are referred. This plane is usually mean sea level and the contours are numbered from this plane upward, all heights being elevations above mean sea level.

In a particular locality that is to be sketched there is generally some point the elevation of which is known. These points may be bench marks of a survey, elevation of a railroad station above sea level, etc. By using such points as the reference point for contours the proper elevations above sea level will be shown.

In case no point of known elevation is at hand the elevation of some point will have to be a.s.sumed and the contours referred to it.

Skill in contouring comes only with practice but by the use of expedients a fairly accurate contoured map can be made. In contouring an area the stream lines and ravines form a framework or skeleton on which the contours are hung more or less like a cobweb. These lines are accurately mapped and their slopes determined and the contours are then sketched in.

If the sketcher desires he may omit determining the slopes of the stream lines and instead determine the elevations of a number of critical points (points where the slope changes) in the area and then draw in the contours remembering that contours bulge downward on slopes and upward on streams lines and ravines.

If time permits both the slopes of the stream lines and the elevation of the critical points may be determined and the resulting sketch will gain in accuracy.

Figs. 5, 6, 7, 8, and 9 show these methods of determining and sketching in contours.

[Ill.u.s.tration: Fig. 5]

[Ill.u.s.tration: Fig. 6]

[Ill.u.s.tration: Fig. 7]

[Ill.u.s.tration: Fig. 8]

[Ill.u.s.tration: Fig. 9]

=1884. Form lines.= It frequently happens that a sketch must be made very hastily and time will not permit of contouring. In this case form lines are used. These lines are exactly like contours except that the elevations and forms of the hills and depressions which they represent are estimated and the sketcher draws the form lines in to indicate the varying forms of the ground as he sees it.

=1885. Scales.= The Army Regulations prescribe a uniform system of scales and contour intervals for military maps, as follows:

Road sketches and extended positions; scale 3 inches to a mile, vertical (or contour) interval, 20 feet.

Position or outpost sketches; scale 6 inches to a mile, vertical (or contour) interval, 10 feet.

This uniform system is a great help in sketching as a given map distance, Par. 1867a, represents the same degree of slope for both the 3 inch to the mile or the 6 inch to the mile scale. The map distances once learned can be applied to a map of either scale and this is of great value in sketching.

Construction of Working Scales

=1886. Working scale.= A _working scale_ is a scale used in making a map. It may be a scale for paces or strides or revolutions of a wheel.

=1887. Length of pace.= The length of a man's pace at a natural walk is about 30 inches, varying somewhat in different men. Each man must determine his own length of pace by walking several times over a known distance. In doing this be sure to take a natural pace. When you know your length of pace you merely count your paces in going over a distance and a simple multiplication of paces by length of pace gives your distance in inches.

In going up and down slopes one's pace varies. On level ground careful pacing will give you distances correct to within 3% or less.

The following tables give length of pace on slopes of 5 degrees to 30 degrees, corresponding to a normal pace on a level of 30.4 inches: