Figure 8-18.-Typical contour formations.CHARACTERISTICS OF CONTOUR LINESA contour line is a line of equal elevation; therefore,two different lines must indicate two differentelevations. So two different contour lines cannotintersect or otherwise contact each other except at apoint where a vertical or overhanging surface, such as avertical or overhanging face of a cliff, exists on theground Figure 8-18 shows an overhanging cliff. Youcan see how the segments of contour lines on this cliffare made as dotted (or hidden) lines. Aside from theexception mentioned, any point where two differentcontour lines intersect would be a point with twodifferent elevations-an obvious impossibility.In forming a mental image of the surfaceconfiguration from a study of contour lines, it is helpfulfor you to remember that a contour line is a level line;that is, a line that would be formed by a horizontal planepassing through the earth at the indicated elevation. Ifyou keep this concept of levelness in mind you canusually get the “feel” of the rise and fall of the groundas you study the contour lines on the map.A contour line must close on itself somewhere—either within or beyond the boundaries of the map. A line8-16
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