One of the troublesome problems in operating a plane table is the difficulty of keeping the alidade blade on the plotted position of the occupied point, such as P in figure 9-9. As the alidade is moved to sight a detail, the edge moves off point P. A solution some-times tried is to use a pin at P and pivot around it, but a progressively larger hole is gouged in the paper with each sight. To eliminate this problem, use two tri-angles to draw a parallel line with the straightedge of the telescope over pivot point P. The small error produced by the eccentric sight is no greater than that resulting from not being exactly over the ground point, P, or even that caused by the telescope axis not being over the edge of the blade.

Other pointers that may be helpful concerning the use of the plane table are as follows:

1. Use buff or green detail paper to lessen the glare.

2. Plot and ink the traverse in advance of the detailing, showing lengths of traverse lines; coordinates of triangulation stations, if known; and useful signals

3. Have a least one vertical control for each three hubs of a traverse, and show all known elevations.

6. Check the orientation on two or more lines if possible.

7. Check the distance and elevation difference in both directions when setting a new hub.

8. Read the distance first and then the vertical angle; or with a Beaman arc, read the H-scale and then the V-scale.

9. To keep the paper cleaner, lift the forward end of the alidade blade to pivot instead of sliding the blade.

11. Check the location of hubs by resection and cutting in (sighting and plotting) prominent objects.

12. Draw short lines at the estimated distances on the map to plot points. Do not start the lines at the hub occupied.

13. Identify points by consecutive numbers or names as they are plotted.

14. Have the rodman make independent sketches on long shots for later transference to the plane-table map.

15. Use walkie-talkie sets to enable the rodmen to describe topographic features when the observer cannot identify them because of distance and obstacles.

16. Use the same points to locate details and contours whenever possible.

17. Sketch contours after three points have been plotted. Points on the maps lose their value if they cannot be identified on the ground.

18. Show spot elevations for summits, sags, bridges, road crossings, and all other critical points.

19. Tie a piece of colored cloth on the stadia rod at the required rod reading to speed work in locating contours by the direct method.

20. Use vertical aerial photographs for plane-table sheets. The planimetric details can be checked and contours added.