694

UNITED STATES COAST AND GEODETIC SURVEY.

(?) Contouring.—After the heights of a sufficient number of points bave been determined to give a good development of tbe terrene tbat is to be mapped, tbe contour lines are drawn in by interpolation between the points of which the élévations had been established.

In a moderately rolling country a limited number of points of known élévations will suffice to draw the contour lines with précision ; but in a rocky région, where abrupt changes and irregular forms predominate, it is almost impossible to plat enough control points to enable the iconometric draftsman to render a faithful représentation of the relief of the broken terrene, and it is here that a close study of the photographs will give the greatest assistance in modifying the courses of the contours to represent the characteristic features of the terrene.

The value of photographie views for a correct or naturalistic délinéation of the topography of a given area is generally acknowledged by experienced topographers, even when using

instrumental methods alone for the control work, as a minute study of the pictured terrene (the photographs) will always aid the draftsman (when inking the topographie sheet) to draw the contours (of which the main deflections had been located instrumentally) with a more natural and artistic reproduction of nature’s forms than could be attained by mechanically inking the penciled lines as obtained solely by instrumental measurements.

Instead of drawing the contour lines at once on the plan, the draftsman may begin by sketching them on the photographs, foliowing the same rules for thefr location as if he were drawing them on the plan, for the image of every platted point is already marked on the photo-graph and its élévation may be taken from the working plan. By following this course he wfll be enabled to follow the inequalities of the surface very closely. Those perspectives of the contours

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