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TOPOGRAPHIC MAPPING Now you know how to perform and record a topographic survey, using the transit-tape or tranSit-stadia methods. Next, we will see how the draftsman (who also might be you) prepares a topographic map. To enhance the explanation of topographic mapping, we will also discuss some additional field methods the surveyor uses.REPRESENTATION OF RELIEF One of the purposes of a topographic map is to depict relief. In fact, this is the main feature that makes a topographic map different from other types of maps. Before you go any further, refresh your memory on the subject of topographic relief. Relief is the term for variance in the vertical configuration of the earths surface. You have seen how relief can be shown in a plotted profile or cross section. These, however, are views on a vertical plane, but a topographic map is a view on a horizontal plane. On a map of this type, relief may be indicated by the following methods.A relief model is a three-dimensional relief presentation-a molded or sculptured model, developed in suitable horizontal and vertical scales, of the hills and valleys in the area.Shading is a pictorial method of showing relief by the use of light and dark areas to suggest the shadows that would be created by parallel rays of light shining across the area at a given angle.Hachures area pictorial method similar to shading except that the light-and-dark pattern is created by short hachure lines, drawn parallel to the steepest slopes. Relative steepness or flatness is suggested by varying the lengths and weights of the lines. Contour lines are lines of equal elevation; that is, each contour line on a map is drawn through a succession of points that are all at the same elevation. A contour is the real-life equivalent; that is, a line of equal elevation on the earths surface. All of these methods of indicating relief are illustrated in figure 8-12. The contour-line method is the one most commonly used on topographic maps.CONTOUR LINES Contour lines indicate a vertical distance above, or below, a datum plane. Contours begin at sea level, normally the zero contour, and each contour line represents an elevation above (or below) sea level. The
Figure 8-12.-Methods of indicating relief. vertical distance between adjacent contour lines is known as the contour interval. Starting at zero elevation the topographer draws every fifth contour line with a heavier line. These are known as index contours. At some place along each index contour, the line is broken and its elevation is given. The contour lines falling between index contours are called intermediate contours. They are drawn with a finer line than the index contours and, usually, do not have their elevations given. Examples of index contours and intermediate contours are shown in figure 8-13. |
 
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