Sources of Error in Plane-Table Work Sources of error in plane-table operation include the following conditions or procedures: 1. 2. 3. 4. 5. 6. Table not level Orientation disturbed during detailing Sights too long for accurate sketching Poor  control Traversing and detailing simultaneously Too  few  points  taken  for  good  sketching Mistakes in Plane-Table Work Some typical mistakes made in plane-table work are  as  follows: 1.  Detailing  without  proper  control 2. Table not level 3.  Orientation  incorrect DEVELOPMENT OF A TOPOGRAPHIC MAP In this final section on topography, we will discuss the  typical  steps  leading  to  the  production  of  a topographic map. In this discussion, you should notice the different operations that are commonly involved and  how  those  operations  interplay  with  one  another. In  developing  a  topographic  map,  you  should  first gather  all  available  maps,  plans,  survey  data,  and utilities data that pertain to the site and study them carefully.  Consider  the  boundaries  of  the  site  in relation to the intended use of the topo map. If the map is  to  be  used  for  design  purposes,  certain  off-site information  will  be  even  more  important  than  on-site details;  for  example,  the  location  and  elevations  of utilities and nearby streets are vital. The location of drainage divides above the site and details of outfall swales and ditches below the site are necessary for the design of the storm drainage facilities. Topographic details  of  an  off-site  strip  of  land  all  around  the proposed limits of construction are necessary so that grading can be designed to blend with adjacent areas. Decide what datum and bench marks are to be used; consider   previous   local   surveys,   U.S.   Coast   and Geodetic  Survey  (USC&GS)  monuments,  sanitary sewer  inverts  (not  rims—they  are  frequently adjusted), and assumed datum. Determine whether there  is  a  coordinate  system  in  the  area  monumented sufficiently for your use; if not, plan to use assumed coordinates. In the latter case, decide on the source of the meridian: adjacent surveys, magnetic, assumed, or shooting the Sun or Polaris (discussed at the EA1 level in Part 2 of this TRAMAN). Next, perform a reconnaissance survey. Observe the vegetation and decide how many men that you, as party chief, will need to cut brush. Select main control traverse  stations  at  points  appropriate  for  plane-table setups. Decide on the number and location of crossties or   secondary   traverse   lines   needed   to   provide sufficient plane-table stations. Select these points so that  plane-table  setups  will  have  to  be  extended  only a  minimum  distance  before  checking  back  into control. The  next  step  is  to  run  the  traverse  lines;  you should check their directions from time to time, where necessary, on long traverses. Checks could be done by astronomical methods (Part 2 of this TRAMAN), by cutoff   lines,   or   by   connecting   the   traverse   with established  points.  Then  run  the  levels,  taking elevation  on  all  traverse  stations.  Close,  balance,  and coordinate  the  main  traverse.  Then  adjust  the  crossties into the main traverse. Balance the levels. Plot the traverse stations by coordinates on the plane-table sheets.  Be  sure  that  each  sheet  overlaps  sufficiently. Also, be sure there is sufficient control on each sheet for  orientation  and  for  extension  of  setups  (if necessary).  Number  the  traverse  stations  with  the same numbers marked on the guard stakes in the field, and  show  the  elevations. The plane-table work is the final big step of the fieldwork, but some transit and level work may still need to be done. The location of some details (such as street  center  lines  or  buildings)  may  need  to  be  more precise than the precision obtainable with the plane table; tie in such details to the traverse by transit tape survey. For design purposes, the elevation of some points (such as the inverts of culverts, paved flumes, sewers, and tops of curbs and gutters) may need to be more precise than the precision obtainable with the plane table. Use the level to obtain such elevations. The final step in the production of the topographic map is, of course, tracing the information from the plane-table  sheets  onto  the  final  drawing. Random traversing, as previously described, is not the  only  way  of  establishing  horizontal  control.  Grids are  frequently  used.  One  good  way  of  identifying  grid lines is to assign a letter to each line in one set and then run  stationing  along  each  line.  Another  method  is described  in  the  paragraphs  below. 9-9