CHAPTER 9 Topographic Maps

CHAPTER 9 Topographic Maps - 2009 Allan Ludman and Stephen...

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CHAPTER 9 © 2009 Allan Ludman and Stephen Marshak WORKING WITH TOPOGRAPHIC MAPS MATERIALS NEEDED • Pencil, ruler, protractor, and tracing paper • Topographic maps provided by your instructor 9.1 INTRODUCTION At the click of a mouse, Google Earth ® and NASA’s World Wind provide satellite images of any point on the planet, Digital Elevation Models (DEMs) help visualize topography, and GIS software can locate points, give elevations, measure lengths of meandering streams, and construct topographic profiles. But geologists still use topographic maps in the field instead of computers because they cost almost nothing, weigh much less than any PDA, and cope with rain, swarming insects, and being dropped better than computers. A recent topographic map gives the names and elevations of lakes, streams, mountains, and roads, and outlines fields and distinguishes swamps and forests as well as a satellite image. With a little practice, you can learn more about landforms from topographic maps than from a satellite image or DEM. This chapter explains how topographic maps work and will help you develop map-reading skills for identifying landforms, planning hikes, and solving environmental problems. And possibly saving your life. 9.2 CONTOUR LINES Like aerial photographs and satellite images, topographic maps show location, distance, and direction very accurately. Topographic maps show the shapes of landforms, elevations, and the steepness of slopes with a special kind of line called contour lines . A contour line is a type of isoline (iso means equal ), a line that connects points of the same value for whatever is being measured. Contour lines can show many types of features on a map, like 1
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population density and average income, as well as elevation. You are already familiar with one common kind of contour line from weather maps where the contours show changes in temperature rather than changes in the elevation of Earth’s surface (Figure 9.1). Figure 9.1: Contour map showing predicted high temperatures for the United States 80º 70º 90º 80º 60º 70º 60º 60º 50º 40º 40º 30º 50º 70º 60º . a. Colored zones show predicted high temperature ranges in b. Contour lines zones outline the same zones increments of 10° F. The contour lines in Figure 9.1 are iso therms (lines of equal temperature). Thus, the predicted temperature for every point on the 60° line is 60°F, every point on the 40° line 40°F, etc. Each isotherm separates areas where the temperature is higher or lower than that along the line itself. Thus, all points in the area north of the 30° contour have predicted high temperatures for the day lower than 30°, those on the south side higher than 30°. The map has a contour interval of 10°, meaning that contour lines represent temperatures at 10° increments.
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CHAPTER 9 Topographic Maps - 2009 Allan Ludman and Stephen...

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