Fig. 320.—Plane table with rollers.
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Fig. 321.—Gurley's plane table adjustment.
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705.—Adjustment of the Plane Table.—There are a great many devices for this. Mr. Pierce, in the admirable paper already mentioned, gives illustrations of the different plans. Some of these have all the complication of the adjustment of the stage of a theodolite, and one has superadded to this a slide-rest motion. These things of course are necessary if the field work is made to take the place of finished office work. One general feature of plane table tripods is some means of adjustment of the table to uneven ground, when the tripod-head cannot be brought nearly level. Gurley's plane table adjustment is perhaps the simplest of any of these devices, and appears to the author to be as good as any other. Fig. 321, D the table top; A a ball fitting turned inside and out, and attached firmly to the table top; C a socket fixed firmly in the head of the tripod; B a bolt with globular head fitting the interior of A, and carried through the head to a winged nut which clamps it firmly. A spring is placed to act against the winged nut, so that when this is slightly loosened the ball fitting A may move between B and C with moderate firmness when the table is being set to an angle.
Fig. 322.—Stanley's high-class plane table.