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Figure  9-8.-Plane-table  notes. - 14070_181
DEVELOPMENT OF A TOPOGRAPHIC MAP - 14070_183

Engineering Aid 1 - Advanced Structural engineering guide book
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PLANE-TABLE  POINTERS 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 Figure 9-9.—Transfer of pivot point. 3. Have a least one vertical control for each three hubs 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 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. 10.  Clean  the  paper  frequently  to  remove  graphite. 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. 21. Use a 6H or harder pencil to avoid smudging. 9-8







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