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Contour

lines, map, interval, scale and line

CONTOUR, kOn'toor, the outline or de fining line of any figure or body; also the hori zontal outline of works of defense. When the conformation of the ground or works is. de scribed by contours or horizontal sections, these sections are taken at some fixed vertical interval from each other suited to the scale of the drawing or the subject in hand, and the dis tances of the surface at each interval above or below some assumed plane of comparison (usually sea-level) are given in figures at the most convenient places on the map. The con tour lines, that is, represent the shore lines that a rise in the ocean would cause it to assume. Of course, embayments of this shore line corre spond to valleys that would be drowned, and promontories to ridges. Contour lines close together mean that a considerable rise in the sea-level would have a comparatively slight effect in pushing back the shore line, which must therefore be situated on very steep ground. As water always flows down hill by the steepest path, and as a line squarely across the contour lines will meet more of them in a given interval than one that meets them slantingly, it is clear that the lines by which the rain is carried off the ground meet all the contour lines at right angles, and that consequently the streams must do likewise. This relation between streams and contour lines, coupled with the fact that streams flow along the bottoms of valleys, and conse quently pass through the inmost points of em bayments in the contour lines, is of the utmost value to the topographer, who always bases his contouring of a region on its drainage.

Two contour lines cannot cross except where there is an overhanging cliff. A contour line cannot terminate; it must either be closed or run off the map. A closed contour line either encloses a hill-top or a depression, which in general has no stream leading from it. Such depressions arc rare except in very dry climates, and even there usually contain a salt lake or marsh, so that confusion between hills and valleys is not likely to arise in reading the map.

It is possible to determine within fairly close limits whether of two points given on a contour map one is visible from the other. This is one reason for the immense superiority of contour maps over all others in military operations.

The contour interval varies according to the scale and purpose of the map and the nature of the terrain. In the United States army, maps with a scale of 12 inches to the mile are sup posed to have a contour interval of five feet; those with a scale of six inches to the mile have a contour interval of 10 feet ; those with a scale of three inches to the mile have a contour interval of 20 feet, and those with a scale of one inch to the mile have a contour interval of 60 feet. This makes the distance between the contour lines dependent only on the mean slope of the ground and not on the scale of the map. In mountainous country, however, the contour intervals may have to be doubled or trebled to obtain a legible map. See MAP ; SURVEYING ; and the map of a part of the Housatonic quad rangle in GEOLOGICAL SURVEY.