Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . is rather difficult of ma-nipulation because of inaccessibility of leveling and clamping-screws, and is in fact too cumbersome for convenient use, ex-cepting where travel is easy. The Johnson plane-table (Fig. 49), so named after its in-ventor, Mr. Willard D. Johnson, is used by the United StatesGeological Survey, and though not quite as rigid as the CoastSurvey type, is sufficiently so for all practical purposes and ismuch lighter, more portable, and more
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . is rather difficult of ma-nipulation because of inaccessibility of leveling and clamping-screws, and is in fact too cumbersome for convenient use, ex-cepting where travel is easy. The Johnson plane-table (Fig. 49), so named after its in-ventor, Mr. Willard D. Johnson, is used by the United StatesGeological Survey, and though not quite as rigid as the CoastSurvey type, is sufficiently so for all practical purposes and ismuch lighter, more portable, and more easily movement is also more compact and less liable to de-rangement or injury. It consists of a split tripod securelyattached to the head as in the case of the Coast Survey tripod,but the leveling and horizontal movements are entirely uniquein surveying-instruments, being essentially an .adaptation ofthe ball-and-socket principle, so made as to furnish the largestpracticable amount of bearing surface. They consist of two cups, one inside the other, the innersurface of the one and the outer surface of the other being. a. Plane-table Board b. Bearing Plate c. Tripod Head d. Legs e. Azimuth Cup /. Upper Level Cupg. Lower h. Level Clamp/. Azimuth Clamp Fig. 50.—Johnson Plane-table Movement. ground to fit as accurately as possible. The interior cup con-sists of two parts or rings, one outside the other, one control-ling the movement in level, and the other that in azimuth (). From each of these there projects beneath the move-ment a screw, and each screw is clamped by a wing cups and rings are bound together and to the tripod TELESCOPIC ALIDADES. 157 head by the two nuts, and are attached to the plane-tableboard by screwing it over a center axis or pin projecting fromthe upper surface of the upper cup. The instrument is firstleveled, not by leveling-screws, but by the ball-and-socket mo-tion given by the pair of cups which are clamped by theupper
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