Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . ading the lighting may be takenfrom one of two directions. If vertical, that is, from above,no high lights are introduced, but the highest summits havethe lightest tint. The better and more commonly acceptedmethod of shading is to assume that the light comes from anangle of 45° to the left, or, in other words, from the upperleft-hand corner of the map; the northwest corner (Figs. 136,138, and 143) and the high lights are, therefore, those whichare tangent
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . ading the lighting may be takenfrom one of two directions. If vertical, that is, from above,no high lights are introduced, but the highest summits havethe lightest tint. The better and more commonly acceptedmethod of shading is to assume that the light comes from anangle of 45° to the left, or, in other words, from the upperleft-hand corner of the map; the northwest corner (Figs. 136,138, and 143) and the high lights are, therefore, those whichare tangent to this direction. Two effective methods of representing relief which aremost useful in sketching in the field on a plane-table boardare: 1. By means of sketch or broken contours; and 2. By means of crayon shading. Sketch contours have the general form of continuous con-tour lines and represent the slopes in plan pictorially. Theyalso give differences of altitude relatively, but the quantityof relief is not accurate over any great space on the map,(Figs. 15, 23, and 137). When the final drawing is made in METHODS OF MAP DRAWING. 453. 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
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