(2) Telemetry. The second method is used in determining long distances for artillery practice and in surveying. It is called telemetry and the use of an instrument is necessary.
(3) Triangulation. This is a long word but one a Scout can learn to know and use. It means that the length of the distance can be computed by means of triangles staked out on the ground, when to measure with a line would be impossible or not satisfactory. It is not necessary to make the sides of the triangles, only the points need to be indicated as it is the relative position of the points which make a triangle and not the lines. These can be marked in the country with poles, stakes or stones; in the city Scouts could stand in position at the necessary points.
When using triangles where shall a Scout place the points?
If the width of a stream, road or field is wanted choose a place where its sides are on about the same level and if possible fairly straight. Then proceed as shown in the accompanying diagram A. Select a conspicuous object on the farther bank of the stream, such as a tree, bush or stone and call it X. Stand opposite it at the near edge of the stream or on the bank, and place a stake A in front of you keeping X and A in direct line, walk backward a few feet and plant a stake B in direct line with them. Right or left face—(for a right angle is necessary at this point). Pace a straight line for say 20 feet and plant a stake C, one high enough to be plainly seen; continue the straight line for say 10 feet more and plant a stake D. Turn inland, (another right angle is here necessary) and pace to the point where the object X on the far side of the stream can be seen in direct line with the stake C. At this point place stake E. Measure the distance from E to D. With paper and pencil mark down the example—for such it is—in this way:
DC : CB :: DE : BX
or
as the length from D to C is to the length of C to B
so
is the length from D to E to the length from B to X
or as in this example,
as 10 is to 20 so 8 is to the distance from B to X, which would be 16. Having discovered the distance between A and B in the case given, to be 4 feet, take this from the distance between B and X and the result will give the width of the stream, which is 12 feet.
Diagram A. To Measure Width of Stream or Road
It may not be always necessary to use the line A—B but if the edge of the stream or road is crooked it is necessary in order to make B—D a straight line at right angles to A—X.
In calculating a height, as that of a tree, house or tower, the triangles can again be used, as shown in diagram B. Choose a level strip of ground; pace the distance in a straight line, from the base of the tree A, or tower, to a point some distance from the tree, and plant a pole or stake say 5 feet high B; continue pacing the straight line to the point where, lying down with eyes level with the tree base, the top of the tree can be seen on a line with the top of the pole; plant here stake C. The height of the tree AA' will be to the length of the distance from C to A as the height of the pole, BB' is to the distance between B and C. A Scout can stand in the place of the stake B.