Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Become familiar with different
ways to portray landforms and
landscapes
■■
Learn the strengths and
weaknesses of these
representations of the Earth’s
surface
■■
Understand the essential
elements of an accurate map
(location, distance, direction,
elevation), how they are
measured, and how they are
portrayed on maps
MATERIALS
NEEDED
■■
Clear plastic ruler with
divisions in tenths of an inch
and millimeters (included in
the Geo Tools section at the
back of this manual)
■■
Circular protractor (included
in the Geo Tools section at the
back of this manual)
■■
A globe and maps provided
by your instructor that show
major cities
8.1 Introduction
Rapidly changing technology makes it easier to study the Earth’s surface today than
at any time in history. We can now make detailed models of the surface from satellite
elevation survey data and download images of any area on the planet with the click of
a mouse. Geologists were quick to understand the scientific value of satellite imaging
technology and adopted new methods as fast as they were developed. Some of the
images in this manual were not even available to researchers a decade ago. Thus the
study of the Earth’s surface today is nearly as dynamic as the surface itself.
This chapter is an introduction to traditional maps and aerial photographs as
well as to some of the newer methods that geologists use to view the Earth’s surface
and understand how its landscapes form. Much of the new technology is free for you
to use on your computer. Google Earth and NASA WorldWind provide free satellite
images of the entire globe and can generate three-dimensional views of landforms.
Digital versions of the newest topographic maps and several generations of historical
maps are available for free for most U.S. states from the U.S. Geological Survey
(USGS) at https://store.usgs.gov. And with Google Maps, you can zoom to maps
showing any location on the Earth.
8.2 Ways to Portray the Earth’s Surface
The ideal way to study landforms would be to first fly over them for a bird’s-eye view
and then walk or drive over them to see them from a human perspective. Since we
can’t do that in this class, we will have to bring the landforms to you instead, using
topographic maps, aerial photographs, satellite images, and digital elevation models.
FIGURE 8.1 portrays an area of eastern Maine using these four different methods.
Topographic maps (Fig. 8.1a) use contour lines (described in Chapter 9) to show
landforms. They used to be drawn by surveyors who measured distances, directions,
and elevations in the field, but they are now made by computers from aerial photographs
and radar satellite elevation data. Aerial photographs (Fig. 8.1b), including
U.S. Geological Survey orthophoto quads, are photographs taken from a plane and
pieced together to form a mosaic of an area. Landsat images (Fig. 8.1c) are made by
a satellite that takes digital images of the Earth’s surface using visible light and other
wavelengths of the electromagnetic spectrum. Scientists can adjust the wavelengths
to color the image artificially and to emphasize specific features; for example, some
infrared wavelengths help reveal the amount and type of vegetation present. Digital
elevation models (DEMs; Fig. 8.1d) are computer-generated, three-dimensional views
of landforms made from radar satellite elevation readings spaced at 10-m or 30-m
intervals on the Earth’s surface. A new generation of DEMs using 1-m intervals is now
being released that provides a more accurate model of the surface than anything
available to the public 5 years ago. Each kind of image has strengths and weaknesses
that make it useful for different purposes, as you will see in Exercise 8.1.
8.2.1 Map Projections
The images in Figure 8.1 are flat, two-dimensional pictures, but the Earth is a nearly
spherical three-dimensional body. Only a three-dimensional representation—
a globe—can accurately show the areas and shapes of the Earth’s features and the
directions and distances between them. The process by which the three-dimensional
Earth is converted to a two-dimensional map is called projection. There are many
different projections, each of which distorts one or more map elements, and each
of which is useful for some purposes but unusable for others. Three common map
198 CHAPTER 8 STUDYING THE EARTH’S LANDFORMS: MAPS AND OTHER TOOLS