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QUESTIONS - CONTINUED - 14071_152
Vertical Control - 14071_154

Engineering Aid 2 - Intermediate Structural engineering guide book
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TOPOGRAPHIC CHAPTER 8 SURVEYING AND MAPPING Topography refers to the characteristics of the land surface.  These  characteristics  include  relief, natural features, and artificial (or man-made) features.  Relief is the conjuration of the earth’s surface and includes such   features   as   hills,   valleys,   plains,   summits, depressions, and other natural features, such as trees, streams, and lakes. Man-made features are highways, bridges, dams, wharfs, buildings, and so forth. A graphic representation of the topography of an area is called a topographic map. A topographic map is simply a drawing that shows the natural and artificial features of an area. A  topographic  survey  is a survey conducted  to  obtain  the  data  needed  for  the  preparation of  a  topographic  map.  This  data  consists  of  the horizontal and vertical locations of the features to be shown on the map. In this chapter and the following chapter, you will study   methods   and   procedures   used   to   perform topographic  surveying  and  to  prepare  topographic maps. TOPOGRAPHIC   SURVEYING The  fieldwork  in  a  topographic  survey  consists principally of (1) the establishment of a basic frame- work  of  horizontally  and  vertically  located  control points  (called  instrument  points  or  stations)  and  (2)  the determination  of  the  horizontal  and  vertical  locations  of details in the vicinity of each instrument point. We will begin  our  discussions  with  topographic  control. TOPOGRAPHIC CONTROL Topographic  control  consists  of  two  parts: (1) horizontal control, which locates the horizontally fixed   position   of   specified   control   points,   and (2) vertical control, in which the elevations of specified bench marks are established. This control provides the framework from which topographic details, such as roads, buildings, rivers, and the elevation of ground points,  are  located. Horizontal Control Locating primary and secondary horizontal control points  or  stations  may  be  accomplished  by  traversing, by triangulation (discussed in part 2 of this TRAMAN), or  by  the  combined  use  of  both  methods.  On  an important, large-area survey, there may be both primary control, in which a number of widely separated primary control  points  are  located  with  a  high  degree  of precision; and secondary control, in which stations are located with less precision within the framework of the primary  control  points. The  routing  of  a  primary  traverse  should  be considered  carefully.  It  should  follow  routes  that  will produce  conveniently  located  stations.  Such  routes might run along roads, ridges, valleys, edges of wooded areas, public land lines, or near the perimeter of tracts of land. This latter route is of particular importance for small areas. When all the details in the area can be conveniently  located  from  stations  on  the  primary traverse, you do not need secondary traverses. However, the size or character of the terrain or both usually make secondary  traverses  necessary.  Consider,  for  example, the situation shown in figure 8-1. This figure shows a tract bounded on three sides by highways and on the fourth side by a fence. For simplification, the figure shows  only  the  items  to  be  discussed.  An  actual complete  plan  would  include  a  title,  date,  scale,  north arrow, and so forth. Figure 8-1.-Primary traverse and secondary traverse. 8-1







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