In contrast the contour lines in the topographic map in Figure 22a indicate a

In contrast the contour lines in the topographic map

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contour lines indicates that the steepness of the slope is rather uniform. In contrast, the contour lines in the topographic map in Figure 2.2a indicate a more linear-shaped hill that is elongated in the north-south direction. The contour lines here are also spaced much closer together on the eastern side of the hill, which tells us that the slope is steeper there. Finally, note the creek in the northwestern part of Figure 2.2a that is represented by the dashed line. Because streams erode and cut downward, they produce ravines and valleys that naturally occupy the lowest parts of the terrain. Notice that when a contour line crosses a stream, it bends or “V’s” in the uphill direction due to the fact the stream is flowing downhill and lies in a topographic low. Figure 2.2 A) Map view of a hill B) Profile view The “rule of V’s” allows one to use contour lines to quickly determine the direction in which the land is rising or falling. Although the direction of slope can always be determined by checking the elevation labels on the contour lines themselves (every fifth line is labeled), using the V’s is much faster. Another quick way to visualize the slope of the terrain is to examine the drainage network shown in blue. For example, find the network of streams in Figure 2.3 that lie on both sides of the road leading to the city of Glennville (this is an actual topographic map produced by the U.S. Geological Survey). Because the stream channels occupy the lowest parts of the terrain, the area between the two drainageways must be a topographic high (i.e., a
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hill). Notice also that the V’s in the contour lines on either side of this high all point toward the top of the hill. Therefore, the best way to visualize the terrain on a topographic map is to examine the blue color of the drainage system and V’s in the contour lines. Note in figure 2.3 that a drainage network often forms a tree-like pattern, where smaller tributaries converge in the downslope direction to form progressively larger streams. Figure 2.3 Map Scale Because topographic maps are scaled down representations of the actual terrain, the scale at which they are drawn is normally listed on the maps themselves. Maps created by the U.S. Geological Survey (USGS) include a type of scale called a representative fraction (RF). The RF is a unitless scale that is written as a proportion, such as 1:24,000. Here the smaller number represents the map distance, whereas the larger number represents the actual ground distance. A user can quickly determine the true ground distance between two points on a map by first measuring the distance with a ruler. This map distance is then multiplied by the map scale. Note that because a RF scale is unitless, the actual distance on the ground will be in the same units as those of the ruler. For example, a distance of one inch on a 1:24,000 map would be equal to 24,000 inches on the ground. Likewise, one centimeter on the same map would represent 24,000 centimeters on the ground. Because inches and centimeters are rather small units, the true ground distance is normally converted into more convenient units, such as miles or kilometers. For example, if the distance on a 1:24,000 map is 5.5 inches, then the actual ground distance in miles would be calculated as follows:
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5.5in
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