CHAPTER 9PLANE-TABLE TOPOGRAPHY AND MAPPROJECTIONIn the previous chapter, you studied the proce-dures used to perform topographic surveying using thetransit-tape or transit-stadia methods. As you know,when either of these methods is used, a topographicmap is prepared as a separate operation that usesthe field notes from the survey to prepare the map.Another method used in topographic surveying andmapping is the plane-table method. This method ispreferred by many surveyors since it combines thefieldwork and the office work into one operation thatproduces a completed, or nearly completed, map in thefield. This chapter discusses the basic principles andprocedures that you will use when performing plane-able topography.Another topic discussed in this chapter is mapprojection. As you will learn, maps can be preparedusing various projection methods to portray all or partof the earth’s surface on the flat plane of a map orchart. As an EA, you will seldom use most of thesemethods in drawing maps. However, it is importantthat you understand the principles of map projectionso that you will be able to read and interpret accuratelythe various types of maps that you will use whenplotting control points for surveys or when plottingfire missions as a mortar platoon member in a con-struction battalion.PLANE-TABLE TOPOGRAPHYAs mentioned above, the plane-table method oftopographic surveying and mapping combines field-work (surveying) with office work (drafting) to pro-duce a topographic map. This is so, because when youuse plane-table equipment, topographic detailsare plotted directly on the map in the field. The plane-able method is advantageous in open country andwhen many irregular lines need to be plotted. It isalso advantageous for small-scale mapping. Thereare, however, some disadvantages. For example, youare required to spend mom time in the field, moreequipment (some awkward to handle) must be carried,and you will need mom time to become skilled inusing the plane table. Other advantages anddisadvantages of the plane-table method are discussedlater in this chapter.A plane-table field party for a large survey shouldconsist of an instrumentman, a note keeper or com-puter, and one or more rodmen. The instrumentmanoperates the plane table and alidade, makes theobservations, and performs the plotting and sketching.The note keeper reduces stadia readings to horizontaland vertical distances and computes the ground eleva-tions for rod observations. He also carries andpositions an umbrella to shade the plane table. Therodman carries a stadia board or Philadelphia rod andholds it vertically at detail points and critical terrainfeatures.Chapter 11 of the EA3 TRAMAN describes theplane-table equipment and uses. That discussionincludes the procedures used to setup and level a planetable and a description of various types of alidades.For plane-table topography, a telescopic alidade,rather than an open-sight alidade, is preferred. Beforeproceeding further in this chapter, it is stronglyrecommended that you review pages 11-33 through11-35 of the EA3 TRAMAN.LOCATING DETAILS BY PLANE TABLEWe will briefly explain the use of the plane tableas follows. Take into the field a sheet of plane-tablepaper of suitable size and which has the controltraverse (fig. 9-1) already plotted to suitable scale.Naturally, you use the same scale as the controltraverse to lay off horizontal distances on the map.Attach the paper to the table. Then set up andcarefully level the table so that D1 on the paper isdirectly over D1 on the ground. In this example, D1 isa point of known elevation (532.4 feet). Now the tablemust be oriented before any detail points can belocated. In other words, the table has to be rotated orturned so that the points plotted on the plane-tablesheet are in relationship to the corresponding pointson the ground. So, with the edge of the alidade bladeon D1 and the telescope trained on A, orient the tableby rotating it to bring D1A on the paper in line with the9-1
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