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 | (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.