PLANE-TABLE POINTERSOne of the troublesome problems in operating aplane table is the difficulty of keeping the alidadeblade on the plotted position of the occupied point,such as P in figure 9-9. As the alidade is moved to sighta detail, the edge moves off point P. A solution some-times tried is to use a pin at P and pivot around it, buta progressively larger hole is gouged in the paper witheach sight. To eliminate this problem, use two tri-angles to draw a parallel line with the straightedge ofthe telescope over pivot point P. The small errorproduced by the eccentric sight is no greater than thatresulting from not being exactly over the ground point,P, or even that caused by the telescope axis not beingover the edge of the blade.Other pointers that may be helpful concerning theuse of the plane table are as follows:1. Use buff or green detail paper to lessen theglare.2. Plot and ink the traverse in advance of thedetailing, showing lengths of traverse lines; coordinatesof triangulation stations, if known; and useful signalsFigure 9-9.—Transfer of pivot point.3. Have a least one vertical control for each threehubs of a traverse, and show all known elevations.4. Cover the portion of the map not being used.5. Setup the table slightly below elbow height.6. Check the orientation on two or more lines ifpossible.7. Check the distance and elevation difference inboth directions when setting a new hub.8. Read the distance first and then the verticalangle; or with a Beaman arc, read the H-scale and thenthe V-scale.9. To keep the paper cleaner, lift the forward endof the alidade blade to pivot instead of sliding the blade.10. Clean the paper frequently to remove graphite.11. Check the location of hubs by resection andcutting in (sighting and plotting) prominent objects.12. Draw short lines at the estimated distances onthe map to plot points. Do not start the lines at the huboccupied.13. Identify points by consecutive numbers ornames as they are plotted.14. Have the rodman make independent sketcheson long shots for later transference to the plane-tablemap.15. Use walkie-talkie sets to enable the rodmen todescribe topographic features when the observer cannotidentify them because of distance and obstacles.16. Use the same points to locate details andcontours whenever possible.17. Sketch contours after three points have beenplotted. Points on the maps lose their value if theycannot 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 atthe required rod reading to speed work in locatingcontours by the direct method.20. Use vertical aerial photographs for plane-tablesheets. The planimetric details can be checked andcontours added.21. Use a 6H or harder pencil to avoid smudging.9-8
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