Author: James A. Moss
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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.

Fig. 1 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 compass 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:

Fig. 2 Fig. 2

(a) From the known or assumed 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 spacing 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.

(Tally Register)—Fig. 3 (Tally Register)—Fig. 3 (Clinometer)—Fig. 4 (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 51/2 divisions; then, since each division is 10 feet, the total height of A above C is 55 feet (51/2 × 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 space (a)-(c), Fig. 1 Y, contours 890, 880, 870, 860 and 850, and they would be spaced 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 repetition 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 assumed 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.

Fig. 5 Fig. 5

Fig. 6 Fig. 6

Fig. 7 Fig. 7

Fig. 8 Fig. 8

Fig. 9 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:

Slopes 0° 5° 10° 15° 20° 25° 30° Length of step ascending 30.4 27.6 24.4 22.1 19.7 17.8 15.0 Length of step descending 30.4 29.2 28.3 27.6 26.4 23.6 19.7

For the same person, the length of step decreases as he becomes tired. To overcome this, ascertain the length of pace when fresh and when tired and use the first scale in the morning and the latter in the afternoon.

The result of the shortening of the pace due to fatigue or going over a slope, is to make the map larger than it should be for a given scale. This is apparent when we consider that we take more paces in covering a given distance than we would were it on a horizontal plane and we were taking our normal pace.

In going up or down a slope of 3 or 4 we actually walk 5 units, but cover only 4 in a horizontal direction. Therefore, we must make allowance when pacing slopes.

In counting paces count each foot as it strikes. In counting strides count only 1 foot as it strikes. A stride is two paces.

In practice it has been found that the scale of strides is far more satisfactory than a scale of paces.

1888. How to make a scale of paces. Having determined the length of our pace, any one of the following three methods may be used in making a working scale:

1st method. The so-called "One thousand unit rule" method is as follows:

Multiply the R. F. (representative fraction) by the number of inches in the unit of measure multiplied by 1000; the result will be the length of line in inches necessary to show 1000 units.

For example, let us suppose that we desire a graphic scale showing 1000 yards, the scale of the map being 3 inches equal 1 mile:

Multiply 1/21120 (R. F.) by 36 (36 inches in 1 yard, the unit of measure) by 1000,—that is,

(1/21120) × 36 × 1000 = 36000/21120 = 1.7046 inches.

Therefore, a line or graphic scale 1.7 inches in length will represent 1000 yards.

If we desire a working scale of paces at 3 inches to the mile, and we have determined that our pace is 31 inches long, we would have (1/21120) × 31 × 1000 = 31000/21120 = 1.467 inches.

We can now lay off this distance and divide it into ten equal parts, and each will give us a 100-pace division.

2nd method. Lay off 100 yards; ascertain how many of your paces are necessary to cover this distance; multiply R. F. by 7,200,000, and divide by the number of paces you take in going 100 yards. The result will be the length of line in inches which will show 2000 of your paces.

3rd method. Construct a scale of convenient

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