3. By use of an instrument called a map measurer, [Fig. 4], set the hand on the face to read zero, roll the small wheel over the distance; now roll the wheel in an opposite direction along the graphical scale, noting the number of yards passed over. Or, having rolled over the distance, note the number of inches on the dial and multiply this by the number of miles or other units per inch. A map measurer is valuable for use in solving map problems in patrolling, advance guard, outpost, etc.
4. Apply a scale of inches to the line to be measured, and multiply this distance by the number of miles per inch shown by the map.
[1866]. Contours. In order to show on a map a correct representation of ground, the depressions and elevations,—that is, the undulations,—must be represented. This is usually done by contours.
Conversationally speaking, a contour is the outline of a figure or body, or the line or lines representing such an outline.
In connection with maps, the word contour is used in these two senses:
1. It is a projection on a horizontal (level) plane (that is, a map) of the line in which a horizontal plane cuts the surface of the ground. In other words, it is a line on a map which shows the route one might follow on the ground and walk on the absolute level. If, for example, you went half way up the side of a hill and, starting there, walked entirely around the hill, neither going up any higher nor down any lower, and you drew a line of the route you had followed, this line would be a contour line and its projection on a horizontal plane (map) would be a contour.
By imagining the surface of the ground being cut by a number of horizontal planes that are the same distance apart, and then projecting (shooting) on a horizontal plane (map) the lines so cut, the elevations and depressions on the ground are represented on the map.
It is important to remember that the imaginary horizontal planes cutting the surface of the ground must be the same distance apart. The distance between the planes is called the contour interval.
2. The word contour is also used in referring to contour line,—that is to say, it is used in referring to the line itself in which a horizontal plane cuts the surface of the ground as well as in referring to the projection of such line on a horizontal plane.
An excellent idea of what is meant by contours and contour-lines can be gotten from [Figs. 5] and [6]. Let us suppose that formerly the island represented in [Figure 5] was entirely under water and that by a sudden disturbance the water of the lake fell until the island stood twenty feet above the water, and that later several other sudden falls of the water, twenty feet each time, occurred, until now the island stands 100 feet out of the lake, and at each of the twenty feet elevations a distinct water line is left. These water lines are perfect contour-lines measured from the surface of the lake as a reference (or datum) plane. [Figure 6] shows the contour-lines in [Figure 5] projected, or shot down, on a horizontal (level) surface. It will be observed that on the gentle slopes, such as F-H ([Fig. 5]), the contours (20, 40) are far apart. But on the steep slopes, as R-O, the contours (20, 40, 60, 80, 100) are close together. Hence, it is seen that contours far apart on a map indicate gentle slopes, and contours close together, steep slopes. It is also seen that the shape of the contours gives an accurate idea of the form of the island. The contours in [Fig. 6] give an exact representation not only of the general form of the island, the two peaks, O and B, the stream, M-N, the Saddle, M, the water shed from F to H, and steep bluff at K, but they also give the slopes of the ground at all points. From this we see that the slopes are directly proportional to the nearness of the contours—that is, the nearer the contours on a map are to one another, the steeper is the slope, and the farther the contours on a map are from one another, the gentler is the slope. A wide space between contours, therefore, represents level ground.