Quantcast CHAPTER 9 PLANE-TABLE TOPOGRAPHY AND MAP PROJECTION - 14071_177

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CHAPTER 9 PLANE-TABLE TOPOGRAPHY AND MAP PROJECTION In the previous chapter, you studied the proce- dures used to perform topographic surveying using the transit-tape or transit-stadia methods. As you know, when either of these methods is used, a topographic map is prepared as a separate operation that uses the field notes from the survey to prepare the map. Another method used in topographic surveying and mapping is the plane-table method. This method is preferred  by  many  surveyors  since  it  combines  the fieldwork and the office work into one operation that produces a completed, or nearly completed, map in the field. This chapter discusses the basic principles and procedures  that  you  will  use  when  performing  plane- able  topography. Another  topic  discussed  in  this  chapter  is  map projection. As you will learn, maps can be prepared using various projection methods to portray all or part of the earth’s surface on the flat plane of a map or chart. As an EA, you will seldom use most of these methods in drawing maps. However, it is important that you understand the principles of map projection so that you will be able to read and interpret accurately the  various  types  of  maps  that  you  will  use  when plotting control points for surveys or when plotting fire missions as a mortar platoon member in a con- struction   battalion. PLANE-TABLE TOPOGRAPHY As mentioned above, the plane-table method of topographic surveying and mapping combines field- work (surveying) with office work (drafting) to pro- duce a topographic map. This is so, because when you use   plane-table   equipment,   topographic   details are plotted directly on the map in the field. The plane- able  method  is  advantageous  in  open  country  and when many irregular lines need to be plotted. It is also  advantageous  for  small-scale  mapping.  There are, however, some disadvantages. For example, you are  required  to  spend  mom  time  in  the  field,  more equipment (some awkward to handle) must be carried, and  you  will  need  mom  time  to  become  skilled  in using   the   plane   table.   Other   advantages   and disadvantages of the plane-table method are discussed later in this chapter. A plane-table field party for a large survey should consist of an instrumentman, a note keeper or com- puter, and one or more rodmen. The instrumentman operates  the  plane  table  and  alidade,  makes  the observations, and performs the plotting and sketching. The  note  keeper  reduces  stadia  readings  to  horizontal and vertical distances and computes the ground eleva- tions  for  rod  observations.  He  also  carries  and positions an umbrella to shade the plane table. The rodman carries a stadia board or Philadelphia rod and holds  it  vertically  at  detail  points  and  critical  terrain features. Chapter 11 of the EA3 TRAMAN describes the plane-table  equipment  and  uses.  That  discussion includes the procedures used to setup and level a plane table and a description of various types of alidades. For  plane-table  topography,  a  telescopic  alidade, rather than an open-sight alidade, is preferred. Before proceeding  further  in  this  chapter,  it  is  strongly recommended that you review pages 11-33 through 11-35 of the EA3 TRAMAN. LOCATING DETAILS BY PLANE TABLE We will briefly explain the use of the plane table as follows. Take into the field a sheet of plane-table paper  of  suitable  size  and  which  has  the  control traverse (fig. 9-1) already plotted to suitable scale. Naturally,  you  use  the  same  scale  as  the  control traverse to lay off horizontal distances on the map. Attach the paper to the table. Then set up and carefully  level  the  table  so  that  D1 on  the  paper  is directly over D1 on the ground. In this example, D1 is a point of known elevation (532.4 feet). Now the table must  be  oriented  before  any  detail  points  can  be located. In other words, the table has to be rotated or turned so that the points plotted on the plane-table sheet are in relationship to the corresponding points on the ground. So, with the edge of the alidade blade on D1 and the telescope trained on A, orient the table by rotating it to bring D1A on the paper in line with the 9-1



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