Contour lines are the means of relating the vertical dimension (the third dimension) of the topography of an area to the two-dimensional surface of a topographic map. Contour lines should be visualized as the intersection of the land surface with a series of equally spaced, horizontal planes that pass through this surface. The vertical distance separating these planes is termed the contour interval (C.I.). Contour intervals are determined by the map scale and the amount of topographic variation within the map area. As viewed on a topographic map, the spacing of contour lines (but not the contour interval) varies according to the changes in slope angle of topographic features (Figure 7.5). Because of this relationship, a few simple Rules of Contour Lines will prove helpful in interpreting the vertical dimension of topographic maps.
- Gentle slopes (low angle from horizontal) on topographic maps with a given interval will be represented by widely spaced contour lines.
- Steep slopes (high angles from the horizontal) on topographic maps with a given contour interval will be represented by closely spaced contour lines. Thus, a vertical cliff large enough to be represented as a topographic feature would be represented by contour lines that merge (i.e., stacked together because of the 90° slope).
- Contour lines that cross streams flowing through valleys of noticeable relief will form a V-shaped deflection with the apex of the V pointing upstream (Figure 7.6). This relationship between contour lines and stream valleys is referred to as the RULE of V's. Because a V will always point upstream, the orientation of V's can always be used to determine the direction of the slope of the surface as well as the direction of the flow of the water.
- Concentric circles of contour lines indicate a hilltop; concentric circles with hachure marks indicate a close depression (Figure 7.6).
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