PLANE TABLEWhen combined with the stadia board orPhiladelphia rod, the plane table are used in what isgenerally known as plane table surveys. Which theseinstruments, the direction, the distance, and thedifference in elevation can be measured and plotteddirectly in the field. The plane table opration producesa completed sketch or map manuscript without the needfor further plotting or computing.A plane table (fig. 11-29) consists of a drawingboard mounted on a tripod with a leveling devicedesigned as part of the board and tripod. The commonlyused leveling head is the ball-and-socket type. Thecross section of a plane table with the tripod head isshown in figure 11-30. The board (G) usually is either18 by 24 in. or 24 by 31 in. and has an attached recessedfitting that screws onto the top of the spindle (A). Awingnut (B) controls the grip of parts C and D on cupE. By releasing the wingnut (B), you can tilt thedrawing board in any direction to level it. Anotherwingnut (F) acts only on the spindle and, when released,permits the leveled board to be rotated on azimuth fororientation. The tripod is shorter than the transit or leveltripods and, when set up, brings the plane table aboutwaist high for easy plotting. One precaution must beobserved in attaching the plane table to the tripod head.A paper gasket should be placed between the fittings toprevent sticking or “freezing” of the threads.The plane table is setup over a point on the groundwhose position has been previously plotted, or will beFigure 11-29.-Plane table.Figure 11-30.-Cross section of a plane table tripod bead.plotted, on the plane table sheet during the operation.The board is oriented either by using a magneticcompass for north-south orientation or by sighting onanother visible point whose position is plotted. Theboard is clamped and the alidade is pointed toward anynew, desired point using the plotted position of the setupground station as a pivot. A line drawn along thestraightedge that is parallel to the line of sight will givethe plotted direction from the setup point to the desiredpoint. Once the distance between the points isdetermined, it is plotted along the line to the specifiedscale. The plotted position represents the new point atthe correct distance and direction from the originalpoint. By holding the plane table orientation andpivoting the alidade around the setup point, you canquickly draw the direction to any number of visiblepoints. The distance to these points is determined byany conventional method that meets the requirementsfor the desired accuracy and can be plotted along theirrespective rays from the setup point. Thus, from onesetup, the positions of a whole series of points can beestablished quickly. For mapping, the difference inelevation is also determined and plotted for each point.The map is completed by subdividing the distancesbetween points with the correct number of contoursspaced to represent the slope of the ground.11-33
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