Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■ Understand how contour
lines on topographic maps
represent the Earth’s surface
■ Practice constructing
topographic profiles to
illustrate and interpret
landforms
■ Use topographic maps to
recognize potential hazards
and solve environmental and
economic problems
MATERIALS
NEEDED
■ Pencil, ruler and protractor
(both included in the
Geo Tools section at the back
of this manual), and tracing
paper
■ Topographic maps provided by
your instructor
■ Graph paper (found at the end
of this chapter)
9.1 Introduction
Today, we have become very familiar with digital tools that help picture the world
around us. With the click of a mouse, Google Earth and NASA WorldWind provide
satellite images of any point on the planet; digital elevation models (DEMs) are
available to help us visualize topography; Global Positioning System (GPS) satellites
circling the Earth can help you locate exactly where you are standing; and
sophisticated geographic information system (GIS) software can locate points, give
elevations, measure lengths of meandering streams, and construct topographic profiles
of an area. But what would you do if you were in the field and did not have
access to any sophisticated equipment or could not get a reliable signal? A geologist
would use a topographic map—a special type of map that uses contour lines to
show landforms. Topographic maps cost almost nothing, weigh much less than a
digital device, and withstand rain, swarming insects, and being dropped better than
computers do. A recent topographic map gives the names and elevations of lakes,
streams, mountains, and roads, and it outlines fields and distinguishes swamps and
forests as well as a satellite image does. 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 helps you develop map-reading
skills for identifying landforms, planning hikes, solving environmental problems—
and possibly, even saving your life.
9.2 Contour Lines
Like aerial photographs and satellite images, topographic maps show location, distance,
and direction very accurately. Topographic maps also show the shapes of
landforms, elevations, and the steepness of slopes with a special kind of line called
a contour line.
A contour line is a line on a map that connects points of the same value for whatever
has been measured: elevations on topographic maps, temperatures on weather
maps (FIG. 9.1), even population densities or average incomes. Thus, the temperature
at every point on the 60° isotherm (temperature contour line) in Figure 9.1 is
60°F, every point on the 40° isotherm is 40°F, and so forth. Each isotherm separates
areas where the temperature is higher or lower than that along the line. Thus, all
points between the 50° and 60° isotherms have temperatures between 50°F and
60°F. The map in Figure 9.1 has a contour interval of 10°, meaning that contour
lines represent temperatures at 10° increments.
FIGURE 9.1 Contour lines on a weather map show
temperature distributions.
70°
80°
50°
60°
40°
90°
60°
60°
80°
70°
40° 30°
50°
70°
9.2.1 Contour Lines on Topographic Maps
Now that you understand the basic concept of contour lines,
let’s look at how they work on topographic maps. A contour
line on a topographic map connects points that have the
same elevation above sea level (sea level is the reference for
all elevations). Topographic maps have contour intervals
chosen to most clearly illustrate the land surface they portray:
small contour intervals are used where there isn’t much
change in elevation, and large contour intervals are used
where variation in elevation is high. Typical contour intervals
used on USGS topographic maps are 10, 20, 50, and
100 feet.
FIGURE 9.2 shows the island of Hawaii mapped with
two different contour intervals, superimposed on a DEM
218 CHAPTER 9 WORKING WITH TOPOGRAPHIC MAPS