Figure 8-31.-Commonly used map symbols.making any attempt to interpolate or to draw thecomplete contour lines. (See fig. 8-30, view B.)Contour lines can be smoothly drawn freehand withuniform width and with best results if a contour pen isused. Breaks in the lines are provided to leave spacesfor the elevations. The numbers that represent theseelevations are written this way so that they maybe readfrom one or two sides of the map. Some authoritiesprefer that elevations also be written in a way that thehighest elevation numbers are arranged from the lowestto the highest uphill. Spot elevations are shown atimportant points, such as road intersections.Figure 8-30, view C, shows the completed contourmap. For more refined work, the EA must trace the map,using a contour pen on tracing paper or other appropriatemedium, to allow reproduction of more copies, ifneeded.Often on a large-scale map, you can represent thetrue shape of features to scale. On small-scale maps,however, you often use symbols for buildings and otherfeatures. Center the symbol on the true position, butdraw it larger than the scale of the map. Detail of thistype is portrayed on the map by means of standardizedtopographic symbols, such as shown in figure 8-31.When you are plotting contours, remember thatstream and ridge lines have a primary influence on thedirection of the contour lines. Also, remember that theslope of the terrain controls the spacing of the contourlines. Contour lines crossing a stream follow the generaldirection of the stream on both sides, then cross thestream in a fairly sharp V that points upstream. Also,remember that contour lines curve around the nose ofridges in the form of a U pointing downhill and crossridge lines at approximate y right angles.INTERPOLATING CONTOUR LINESIn the examples of interpolation previously given, asingle contour line was interpolated between two pointsof known elevation, a known horizontal distance apart,and by mathematical computation. In actual practice,usual] y more than one line must be interpolated betweena pair of points; and large numbers of lines must beinterpolated between many pairs of points.Mathematical computation for the location of each linewould be time-consuming and would be used only in asituation where contour lines had to be located with anunusually high degree of accuracy.For most ordinary contour-line drawings, one ofseveral rapid methods of interpolation is used. In eachcase it is assumed that the slope between the two pointsof known elevation is uniform.Figure 8-32 shows the use of an engineer’s scale tointerpolate the contours at 2-foot intervals between Aand B. The difference in elevation between A and B is8-22
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