Laboratory Manual for Introductory Geology 4e

this information on the most recent USGS topographic maps, and Appendix 9.1
shows standard map colors and symbols used to represent natural and artificial features.
Digital versions of the newest USGS topographic maps are available (free)
at https://store.usgs.gov and contain recent aerial photographs as well as the
traditional map.
9.3.1 Contour Lines and Slope
A DEM uses shading and perspective to show the configuration of the land surface,
whereas a topographic map uses contour lines. The best way to understand
how contour lines portray hills, valleys, rivers, and other landforms is to compare a
topographic map (FIG. 9.4) with a DEM (FIG. 9.5) on which the same landscape
is clearly visible. There are rules for reading contour lines that will allow you to see
how they can provide accurate images of complex three-dimensional landforms.
The first four exercises in this chapter will help you learn those rules, starting with
how contour lines portray the slopes of hills and valleys.
EXERCISE 9.1
How Topographic Maps Show Steep and Gentle Slopes
Name:
Section:
Course:
Date:
Compare the topographic map (Fig. 9.4) and the DEM (Fig. 9.5) of a part of eastern Maine.
(a) Slope steepness: On the DEM, find a flat space and a place where the slope is steep. Now locate these places on
the topographic map. Compare the contour lines in the flat and steep places.
●
How does the spacing of the contour lines show the difference between gentle and steep slopes?
●
Describe the slopes on both sides of Farrow Lake, the first lake west of Topsfield. Do the slopes on the east and west
sides of the lake have the same steepness, or is one steeper than the other? Explain your reasoning.
●
Describe the steepness of the slopes of Farrow Mountain in words, and draw a sketch showing what it would look like
to climb over the mountain from northwest to southeast.
(b) Nested contour lines: The colored circles on the topographic map identify places where there are a series of
concentric (nested) contour lines. Look at these features on the DEM.
●
What type of landform do nested contour lines indicate?
●
Mark similar features on the topographic map with a colored pencil or marker.
220 CHAPTER 9 WORKING WITH TOPOGRAPHIC MAPS