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a.b. = vertical interval corresponding to c.d. Am=fb.
" These Contours should have been represented by dotted lines".
Scale 2 inches to 1 mile
intermediate contours filled in by the eye; to perform which with tolerable accuracy, with the assistance of the instrumental contours previously marked by pickets on the ground, becomes, after a little practice, an operation of no great difficulty.
Even in surveys where the delineation of the surface of the ground is to be represented entirely by sketching on the horizontal system, as described in page 60, a few distant instrumental contours very much facilitate the work, and give it a character of truth and certainty that could not otherwise be looked for.
Fig. 1, Plate 8, illustrates the method of tracing and surveying the contour lines when the operation is carried on between the separate secondary triangles on an extensive survey. As has been remarked however, there is no necessity for following this system of working rigidly within the boundary lines of these triangles, as bench-marks established at any convenient spots out of the direct line connecting two trigonometrical stations, answer just as well for checks upon the progress of the work, and for datum points from whence to commence, and upon which to close the work.
Supposing, for instance, the altitudes of the trigonometrical points B, C, D, had been previously ascertained to be respectively 625, 570, and 282 feet above the level of the sea, and that the instrumental contours were required to be marked at equal vertical intervals of 50 feet above that level. Starting from either of these points, say C, in the direction of CB, mark the level of the nearest line of contours, which in this case would be 20 feet below C; and then the points where every difference of altitude of 50 feet would cut the line C B (500, 450, &c.). On arriving at B a check is at once obtained upon the section that has just been run; and the error, if any, can be corrected upon the spot. The other sides of the triangle, B D and D C, are then levelled in the same manner; the connection of the corresponding contour lines cutting each of them traced out by the spirit-level; and their position in plan laid down, either by traversing, or by reference to points and lines already surveyed and plotted. The places of many of these contour pickets can generally be ascertained whilst the levelling is in progress, by measuring their distances from the instrument, and observing the angles made by them and the trigonometrical or other