Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . the board.(Fig. 54.) The alidade used with this instrument consists ofa simple straight-edge of brass, 18 inches in length, with fold-ing sights, the foresight being a slot with two or three peep-holes. With this apparatus the writer has carried a systemof plane-table triangulation, accompanied by vertical andhorizontal angles with the gradienter, and has made a com-plete geographic map on a scale of 4 miles to i inch and withsketched contours of 200 feet
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . the board.(Fig. 54.) The alidade used with this instrument consists ofa simple straight-edge of brass, 18 inches in length, with fold-ing sights, the foresight being a slot with two or three peep-holes. With this apparatus the writer has carried a systemof plane-table triangulation, accompanied by vertical andhorizontal angles with the gradienter, and has made a com-plete geographic map on a scale of 4 miles to i inch and withsketched contours of 200 feet interval, in a season of sevenmonths, over an area of 11,000 square miles. The extremeerror of location on the plane-table, as afterwards correctedby the gradienter angles platted to a primary theodolite tri-angulation, was a little in excess of \ inch in a linear distanceof 140 miles. 64. Cavalry Sketch-board.—This is a modified plane-table devised by Captain Willoughby Verner of the BritishArmy. It has an extreme length of 9 to 12 inches and anextreme width of 7 to 9 inches. (Fig. 55.) On either side CA VA LR Y SKE TCH-BOAKD. 165. Fig. 55.—Cavalry Skeichboard and Straight-edge. l66 PLANE-TABLES AND ALIDADES. are two rollers held by friction thumb-screws over which acontinuous roll of paper is passed. At one end of the boardis a declinatoire or small box compass, while on its under sideis a pivoted strap by which it can be fastened to the wrist ofthe surveyor and revolved for orientation. This apparatus isused chiefly as a traverse plane-table board, the line of direc-tion through the compass being parallel to the general direc-tion of the route traversed. An attachment to the under sideof the board permits of its being fastened to a light tripod orJacobs-staff, when desired. An adjunct to its use is a lightalidade with scale, and it is employed much as is a plane-table excepting that its range is limited by the angle seenahead when attached to the wrist. Instead of a decl
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