CHAPTER XV.
GRAPHIC SURVEYING INSTRUMENTS AND APPLIANCES CONNECTED THEREWITH—PLANE TABLES—ALIDADES—TELESCOPIC ARRANGEMENTS—SUBTENSE MEASUREMENTS—VARIOUS DEVICES FOR HOLDING THE PAPER—CONTINUOUS PAPERS—ADJUSTMENT OF TRIPOD HEADS—METHOD OF USING—EDGEWORTH'S STADIOMETER—SKETCHING PROTRACTOR—SKETCHING CASE—CAMERA LUCIDA, ETC.
697.—Plane Tables.—These instruments have been used for filling in the greater number of topographical surveys in all countries. They possess the merit that any intelligent, untrained person can be readily brought to comprehend their manipulation in the work to be performed, as angles of position of objects are taken directly by drawing lines pointing to them from a point upon a sheet of paper stretched upon a table. In new countries natural objects without very marked outline are conveniently defined for position. The objection to this method, from a point of view of the practical surveyor, is that the work which can be done with equal facility in a comfortable office from the field-book is with this instrument performed in the open air, under risk of rain, dust, and other atmospheric discomforts affecting both the person and the material on which he works. But for countries where the climate can be depended upon, the facility with which surveyors with little experience can map details for filling in superior triangulations made with the theodolite, its use has gained much favour. Natives can be easily taught to use it, and the check on their work through the previous triangulation is perfect. The subject of plane tables will in these pages be considered only in its general aspect, with the examples of a few good instruments, referring the reader who cares to follow the subject further to an excellent paper by Mr. J. Pierce, Jun., read before the Institute of Civil Engineers, February, 1888.[51]
698.—The Plane Table in its simplest form consists of a small drawing-board mounted upon a firm tripod stand, and is shown at Fig. 313.
Fig. 313.—Simple plane table.
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A rule termed an alidade, with sights placed at its ends, gives the direction of any object from a given point on the sheet of paper stretched upon the table, to which a fine line is drawn by an HH pencil to point the direction. The alidade sometimes carries a trough compass fixed upon it, but this is generally a separate instrument which is placed against its fiducial or ruling edge to give a magnetic north to south line, to which all other lines are assumed to take angular direction. A loose spirit level is also provided, by means of which the board may be set level by shifting the legs of the tripod.
699.—Plane Table with Telescope.—Where greater refinement of observation is required than is possible with sights, a telescope is mounted on the alidade, which moves in the vertical plane upon an axis, so that it may be directed in a linear direction with the fiducial edge of the rule to any point in azimuth. The telescope sometimes carries a level, so that the table may be set level by means of the alidade.
