Manual of Military Training / Second, Revised Edition

by James A. Moss

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 compass 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.) Assume 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.

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: