Fig. 314.—Plane table.
Fig. 315.—Tripod stand.
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700.—A class of plane table which meets all necessary refinement for ordinary filling in of field work is shown in the illustration, Fig. 314. This nearly resembles those made by the author for filling in details of the great trigonometrical survey of India. The drawing surface of this table consists of a loose panel which stretches the sheet of paper by pressing it into its frame, where it is afterwards held by a pair of ledges which fit at their ends into long slots. The panel of the board, shown in detail Fig. 316, is mounted upon a firmly braced tripod stand. The head of the tripod stand, shown Fig. 315, is secured to the board with a central screw (not shown) which permits the board to be set in any direction, it being the rule that the edge W should always take a north to south direction. Three screws sss at the corners of the triangular head can be raised or lowered by milled heads from the under side. These screws permit about 15° of adjustment to the table without any unsteadiness, as the centre screw clamps it finally hard down upon them when all adjustments are made. A small trough form of magnetic compass a is placed upon the rule to strike the magnetic north to south line, to which all angles are referred in transposing the work of the plane table. The diaphragm of the telescope is provided with a platino-iridium point fixed vertically at the mutual focus of the object-glass and the eye-piece. A pair of points to subtend an angle to measure a staff for distance, [Fig. 319], is a convenient addition.
Fig. 316.—Panel board of plane table.
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701.—The Telescopic Arrangement of the alidade is varied in different countries. In some cases it is placed near to one end, which is perhaps better than in the centre of the rule, as it is more easily read. In the modern French military alidade a prismatic eye-piece is used, so that observation is made by looking directly down upon the eye-piece of the telescope. In the Prussian alidade adjustment is made to the standard of the telescope so as to bring the horizontal axis upon which it moves level, that the telescope may move in azimuth, however irregular or uneven the surface of the paper on the board may be. This is necessary for any great degree of refinement in the plane table, as the surface of a piece of wood upon which the paper is stretched will be almost certain to warp if exposed to all weathers, and this, added to the small width of the alidade, can scarcely retain the axis in exact horizontality, placed as it is high above the surface of the table. Some plane tables made by the author for General Robinson for Indian service were of papier-maché to remedy the defect of warping, but even this material warps upon exposure. Plane tables have been made in Germany of metal and of glass, but in this case the weight is a great objection. The author has found surfaced slate very good, but it has the same objection of too much weight for a portable instrument.
Fig. 317.—Stanley's plane table.