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To meet all conditions the revisor has designed a high-class plane table shown at Fig. 322. This has a ball and socket rough levelling arrangement and parallel screws for fine levelling, circular motion to table with clamp and tangent, spring rollers for taking any length of paper and instantly clamping it, alidade with extra powerful telescope with vertical circle divided on silver reading by two verniers to minutes with clamp and tangent motion, cross levels, diagonal scale, and adjustment for setting telescope to revolve in vertical plane, circular compass with cross levels and plumbing bar. The board is generally made 30″ × 24″. The telescope is stadia reading and is made with long sensitive bubble mounted upon it or upon the verniers if preferred as shown at Fig. 323.
Fig. 323.—Stanley's high-class alidade with bubble on verniers.
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706.—Method of Using the Plane Table.—The table is first set and levelled up at a commanding position to observe the extent of country it is intended to plot from observation from a single station. Let Fig. 324 1 be the first station for plotting the enclosure abcdef. Draw lines by the alidade pointing to these angles represented by the letters from a point near 1. Set up a picket or stadium at the station 2 where it is intended that the plane table shall next be set up, and draw the line 1 2 distinctly on the paper. Measure the line 1 2 either by its subtense on the stadium or by direct chain measurement, and plot this from station 1 on the paper according to the scale to be worked to in making the plan. On removing the table set up a picket or distinct land mark vertical with the position of station 1 occupied on the paper. Move the table to station 2 at the measured distance and set the direction of the board by means of the alidade so that the line 2 1 cuts the picket left at station 1. Now draw lines from station 2 to all the points abcde, cutting the former lines as represented by dotted lines in the figure, and the intersections of these lines will give the true positions of abcde, according to the scale selected for the base 1 2, and these may be tied up to represent the boundaries, as shown on the plane table 2. It will be readily seen that the line 1 2 represents a bearing in azimuth; so that if the edge of the table be set, say truly N. to S., in both positions the line on the paper 1 2 will agree in both these positions of the table; but the check by the alidade of this line is valuable to save risk of error.
Fig. 324.—Diagram of plane table work.
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707.—Where an extent of land is to be surveyed by the plane table, longitudinal bands of a mile or so in width are taken. Where the roller plane table with continuous paper, [Fig. 320], is used, the forward points of observation are lined in and the backward ones simply tied up, being certain by observations written in pencil upon the paper that identical objects are tied up from the positions of both stations. Where an object cannot be seen from both stations its position may be indicated by the stadium from a single bearing, or it may possibly be tied up from a further advanced station.