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CONTOUR MAP CONSTRUCTION If EAs can perform ordinary engineering drafting chores, they will not have any difficulty in constructing a topographic map. To some degree, topographers must draw contour lines by estimation. Their knowledge of contour line characteristics and the configuration of the terrain that the contour lines represent will be a great help. Topographers must use their skill and judgment to draw the contour lines so that the lines are the best representation of the actual configuration of the ground surface.Basically, the construction of a contour map consists of three operations. They are as follows:1. Plot horizontal control that will serve as the framework of the map.2. Plot details, including the map location of ground points of known ground elevation. These ground
Figure 8-30.Plotting detail and contouring. points or contour points will be used as guides for the proper location of the contour lines.3. Construct contour lines at given contour intervals.Take special care, in the field, to locate ridge and valley lines because you usually draw these lines first on the map before plotting the actual contour points. (See fig. 8-30, view A.) Since contours ordinarily change direction sharply where they cross these lines and the slopes of ridges and valleys are fairly uniform, these lines aid you in drawing the correct contour lines. After the ridge and valley lines are plotted, space contour crossings (by interpolation) along them before
Figure 8-31.-Commonly used map symbols. making any attempt to interpolate or to draw the complete contour lines. (See fig. 8-30, view B.) Contour lines can be smoothly drawn freehand with uniform width and with best results if a contour pen is used. Breaks in the lines are provided to leave spaces for the elevations. The numbers that represent these elevations are written this way so that they maybe read from one or two sides of the map. Some authorities prefer that elevations also be written in a way that the highest elevation numbers are arranged from the lowest to the highest uphill. Spot elevations are shown at important points, such as road intersections.Figure 8-30, view C, shows the completed contour map. For more refined work, the EA must trace the map, using a contour pen on tracing paper or other appropriate medium, to allow reproduction of more copies, if needed. Often on a large-scale map, you can represent the true shape of features to scale. On small-scale maps, however, you often use symbols for buildings and other features. Center the symbol on the true position, but draw it larger than the scale of the map. Detail of this type is portrayed on the map by means of standardized topographic symbols, such as shown in figure 8-31. When you are plotting contours, remember that stream and ridge lines have a primary influence on the direction of the contour lines. Also, remember that the slope of the terrain controls the spacing of the contour lines. Contour lines crossing a stream follow the general direction of the stream on both sides, then cross the stream in a fairly sharp V that points upstream. Also, remember that contour lines curve around the nose of ridges in the form of a U pointing downhill and cross ridge lines at approximate y right angles. |
 
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