elevations from vertical angulation or spirit-leveling written in their appropriate places. If the work be the making of a topographic map on scales larger than those above described, and the country be still of the same topographic character-namely, open, with salient summits, -a system of control similar to the above must in like manner first be executed by the development of plane-table triangulation and the running of control, traverse, and level lines. But the after-work of sketching the map will be conducted in a different manner than for the smaller scales, because of the greater detail required, the shorter distances to be traveled by the topographer in performing the work, and his consequent nearness to the various features which he is to map. 13. Sketching Open Country. Having the control. platted on the sketch sheet as shown in Fig. 3, and where roads are sufficiently abundant to cut up the map with traverses so near one to the other that the topographer may not have to sketch more than one-half to one inch to either side of his position, the sketching of the topography proceeds as follows: Taking the sketch sheet on a board in his lap, the topog rapher for cheapness and convenience, because of the speed, drives over every road. Where these are not sufficiently near one to the other he walks in between them, pacing distance (Art. 95), and getting direction by sighting fixed objects, while he sketches the plan of the contour lines (Art. 193) as far as he can safely see them to either side of his path. This operation is performed by setting out from such fixed points as a road junction, a located house, or a stream crossing, the position of which is platted on his map and the elevation of which is known. Adjusting the index of his aneroid at the known elevation (Art. 176), he drives along, keeping the platted direction of the road parallel to its position on the ground and marking on the map the positions at which the SKETCHING OPEN COUNTRY. 29 Thus, if his contour various contours are crossed by his route. interval be twenty feet, at every change of twenty feet as indicated by the barometer he stops, and, knowing his position. on the map either by reference to bends in the roads, houses, or by having counted the revolutions of his wheel from a known point, he glances along the trend of the slopes to one side or the other, following by eye the level line of his contour, and this he sketches in horizontal plan upon the map. At first he may be aided in this by a hand-level (Art. 156), but as he acquires skill with practice he is able to estimate the position and direction of the contour line to either side with great accuracy, and finally to interpolate other contours above and below that on which he is placed with such precision as not to affect the quality of the resulting map by a contour interval. The aneroid being an unreliable instrument, he must not drive more than two or three miles without checking it at a well-determined elevation. This he is usually able to do at houses, or hill-summits, or other points the positions of which have been determined by his prior control. If he is not able so to check his aneroid, he hastily sets up his plane-table, reads with the telescopic alidade a few vertical angles (Art. 162) to hilltops or houses in sight and the elevations of which are known, and, with these angles and the distances which he can measure from his position to the points sighted as shown on the adjusted control, he is at once able to compute the elevation of his position (Art. 161) within two or three feet and thus check his aneroid. At the same time he is in similar manner able frequently to throw out other elevations by sighting from the position thus determined to houses or summits near by which may have been located by the traverse (Art. 84), and the heights of which he determines now from his angulation. The topographer thus sketches the whole area assigned him, not only mapping the contours, drainage, political boundaries, and other topographic features, but also |