When combined with the stadia board or Philadelphia rod, the plane table are used in what is generally known as plane table surveys. Which these instruments, the direction, the distance, and the difference in elevation can be measured and plotted directly in the field. The plane table opration produces a completed sketch or map manuscript without the need for further plotting or computing.

A plane table (fig. 11-29) consists of a drawing board mounted on a tripod with a leveling device designed as part of the board and tripod. The commonly used leveling head is the ball-and-socket type. The cross section of a plane table with the tripod head is shown in figure 11-30. The board (G) usually is either 18 by 24 in. or 24 by 31 in. and has an attached recessed fitting that screws onto the top of the spindle (A). A wingnut (B) controls the grip of parts C and D on cup E. By releasing the wingnut (B), you can tilt the drawing board in any direction to level it. Another wingnut (F) acts only on the spindle and, when released, permits the leveled board to be rotated on azimuth for orientation. The tripod is shorter than the transit or level tripods and, when set up, brings the plane table about waist high for easy plotting. One precaution must be observed in attaching the plane table to the tripod head.

A paper gasket should be placed between the fittings to prevent sticking or "freezing" of the threads. The plane table is setup over a point on the ground whose position has been previously plotted, or will be

plotted, on the plane table sheet during the operation. The board is oriented either by using a magnetic compass for north-south orientation or by sighting on another visible point whose position is plotted. The board is clamped and the alidade is pointed toward any new, desired point using the plotted position of the setup ground station as a pivot. A line drawn along the straightedge that is parallel to the line of sight will give the plotted direction from the setup point to the desired point. Once the distance between the points is determined, it is plotted along the line to the specified scale. The plotted position represents the new point at the correct distance and direction from the original point. By holding the plane table orientation and pivoting the alidade around the setup point, you can quickly draw the direction to any number of visible points. The distance to these points is determined by any conventional method that meets the requirements for the desired accuracy and can be plotted along their respective rays from the setup point. Thus, from one setup, the positions of a whole series of points can be established quickly. For mapping, the difference in elevation is also determined and plotted for each point. The map is completed by subdividing the distances between points with the correct number of contours spaced to represent the slope of the ground.

The alidade (fig. 11-31) is a straightedge with a sighting device parallel to the edge. The more precise types have telescopes for sighting, special retitles for measuring distance, and graduated arcs for measuring vertical angles. A new version also includes a self-leveling, optical-reading system with enclosed graduated arcs.

1. The open-sight alidade (fig. 11-31, view A), which is very useful in sketching small areas, has a collapsible open sight attached to a straightedge. A level bubble is mounted on the straightedge for keeping the alidade level. A trough compass is also furnished for attaching to the sketch board. By sighting through the peep sight, the operator can determine a level line and the slope from the sighting point. No magnification is provided, so the sight lines are kept comparatively short. The distances can be estimated by pacing or can be measured with a tape if more accuracy is required. A 10-mil graduation that is numbered every fifth tick mark from 0 to 40 runs up on the right edge and down on the left edge of the front sight for determining slopes.

2. The telescopic alidades (fig. 11-31, views B and C) consist of straightedges with rigidly mounted telescopes that can be rotated through a vertical angle of 30 0 . One type has a telescope set on a high standard or post to raise it above the table. This permits direct viewing through the telescope, which is at a comfortable height. The other type has the telescope mounted close to the straightedge. A right-angle prism is attached to the eyepiece and permits viewing through the telescope by looking down into the eyepiece prism.

3. The telescope for the high standard is 16 power; for the low standard, 12 power. Both are the inverting type with internal focusing. The prismatic eyepiece inverts the image top to bottom, so that it appears erect but reversed side to side. The line of sight through the telescopes in a level position is parallel to the straightedge on the base. The telescope reticle has horizontal and vertical cross hairs and a set of stadia hairs. As you already knew, the stadia hairs are used to measure distances. The vertical distance between the upper and lower stadia hairs is carefully read and multiplied by the stadia interval factor. This value is the straight-line distance between the instrument and the rod.

4. A circular bubble and a magnetic compass needle are attached to the base. These are used to level the plane table and orient it to its proper position. Since the ball-and-socket head does not permit as fine a movement as the leveling screw, the bubble is centered as accurately as possible. Then, the wingnut (fig. 11-30, view B) is set firmly but not tightly. When the plane table is tapped lightly on the proper corner, the operator can refine the leveling and then properly tighten the wingnut. To orient the plane table, loosen wingnut F and rotate the table. It is a good practice to draw a magnetic north line on the cover sheet or on two pieces of tape attached near the edges of the board. The straightedge is set on this line during orientation. When the plane table is rotated to face north, the magnetic needle is released and will have room to swing in its case without hitting the sides.

5. The telescopic alidades have two other important features used for plane table surveying. These are the detachable striding level and the

Figure 11-31.-Types of alidades.

stadia arc. The striding level contains a long bubble, and when attached, permits accurate leveling of the line of sight. The bubble is mounted on a metal tube with V-fittings on each end. The fittings are placed astride the telescope and bear on built-in polished brass rings on each side of the center post. A spring clip on the level grips a center pin on top of the telescope and keeps the level from falling or being knocked off during operation. A button on the side of the level releases the clip for removing the level. For checking and adjusting, the level is reversible. The striding level normally is used to establish a horizontal line of sight and to use the alidade as a level. The stadia arc assembly consists of a vertical arc mounted on the end of the left trunnion and a vernier attached to the left bearing by an arm. A level vial is attached to the upper end of the arm; a tangent screw controls the movement of the vial. Once adjusted, this vial establishes a reference from which vertical angles can be measured even if the plane table is not exactly level. The stadia arc is a vertical scale attached to the alidade. With the stadia arc, it is possible to determine horizontal distances and differences in elevation by the stadia method.

6. A new model telescopic alidade is the self-leveling, optical-reading instrument. Instead of the exterior arc and level bubble, a prism system with a suspended element and enclosed arcs is used. As long as the alidade base is leveled to within one-half degree of horizontal, the suspending element (or pendulum) will swing into position. Then the vertical arc index that is attached to it will assume a leveled position. The scales are read directly through an optical train. This combination permits faster operation. In addition, there is no chance of forgetting to index the arc bubble and introducing errors into the readings.

Some of the auxiliary equipment used with a plane table consists of a coated plastic or a paper plane table sheet on which the map or sketch is drawn, drawing materials (scribing tools for coated plastic or pencils for the paper), scales for plotting distances, triangles, waterproof table covers, umbrella, and notebook. The plane table sheet is attached to the board by flatheaded, threaded studs that fit into recesses in the table and do not obstruct the alidades movement.