Topographical surveying and sketching by Rees Thomas Henry

Author:Rees, Thomas Henry
Language: eng
Format: epub
Tags: Topographical surveying, Topographical drawing
Publisher: [Leavenworth, Kan., Ketcheson Printing Company
Published: 1908-03-25T05:00:00+00:00

r.

'S' /

^Ure c

0

.^

r-^

\

-^^^

CHAPTER IX.

THE PLANE TABLE.

The plane table is a surveying instrument con-' sisting of a drawing board supported on a tripod, and a ruler to which a pair of sights or a telescope is attached. It is used in the field for projecting the lines and points of a survey directly upon the drawing, and thus it eliminates the measuring of horizontal angles and the use of the protractor. The transit or theodolite, the sextant and the compass are used to measure angles, and thus determine directions. The plane table is used to determine directions with-out measuring angles. Since its purpose is the same, it is generally classed with the angle measuring instruments.

DESCRIPTION.

The board should not be larger than 24x30 inches. A larg-e board is difficult to keep level, and is heavy to carry. A smaller board, say 18x22 inches, is better if it fulfills the purpose of the survey. The board should be made of light material (white pine is best) and should be so constructed as to prevent warping and splitting.

The drawing paper should be fastened to the board by means of screws with flat heads. Metal sockets for these screws are let into the wood at the corners

of tiie board, and at these points the board is countersunk so that the top of the screw head shall be flush with the surface of the board. The ruler can then be placed anywhere on the board without interference. Other means of fastening the paper are thumb tacks and spring clips, but these interfere with the ruler in some positions. Thumb tacks also mar the board. Rollers at the ends of the board, are sometimes used for fastening and stretching the paper.

The board rests on a leveling head, to which it is fastened by screws with a connection that permits the board to be revolved in its own plane or to be clamped in a fixed position. Various forms of leveling heads are used for the plane table. The most accurate method of leveling is by means of leveling screws like those of the transit.

The Johnson plane table movement is compact and simple. (Plate B, Fig. 2.) The top of the tripod is a spherical cup (A) with a large hole in the bottom. The board (G) is fastened to a similar cup (B) which nests into the first and forms a universal joint which permits the board to be leveled and turned in azimuth.

These motions are clamped by means of a double spindle (CD) that passes through the hole in the lower cup. A washer (E) and wing-nut (F) on the outer spindle (C) clamp the leveling motion, while leaving the board free to turn in azimuth. A wing, nut (H)on the end of the inner spindle (D) clamps the azimuth motion. The large spherical surface of contact at the joint gives great rigidity and firmness to the connection. A slow motion tangent screw cannot be applied to this device.

The alidade of the plane table consists of the ruler with its telescope or sights.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.