Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . u CONTOUR LINES. 455 office from such a sketch map, the altitudes which have beendetermined everywhere give points upon which connectedcontour Hues can be constructed by following the forms in-dicated by the sketch contours. Crayon or briisJi shading bears about the same relation tohachure drawing as do sketch contours to continuous contour. Fig. 137.—Sketch Contours. Xalapa, Mexico. lines. By the means of a soft crayon or pencil the shapesand steepness of
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . u CONTOUR LINES. 455 office from such a sketch map, the altitudes which have beendetermined everywhere give points upon which connectedcontour Hues can be constructed by following the forms in-dicated by the sketch contours. Crayon or briisJi shading bears about the same relation tohachure drawing as do sketch contours to continuous contour. Fig. 137.—Sketch Contours. Xalapa, Mexico. lines. By the means of a soft crayon or pencil the shapesand steepness of slope of the terrane can rapidly be repre-sented in the field, and, if it is desired, the same can afterwardsbe worked up into a finished hachure map, or, providing ele-vations are sufficiently abundant, into a contour map. 192. Contour Lines.—In order to represent the terranequantitatively, that is, to show not only the slopes and shapesof the country and the relative steepness of the hills, but 45^ TOPOGRAPHIC DRAWING. also amounts and differences in elevation at any point, asystem of map construction is employed called lines are lines of equal elevation above some datumas the mean sea-level. They are drawn at regular verticalintervals, their distances apart being dependent upon thehorizontal scale of the map, and they thus indicate not onlyactual differences of elevation, but degrees of steepness orgrades. Contour lines express three degrees of
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