TOPOGRAPHICAL SURVEY

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.

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 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 makes 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