Geology Reference
In-Depth Information
FIGURE 2.11
West-east cross section of the Michigan Basin from NW of Milwaukee, Wisconsin (A), to NE of London, Ontario, Canada
(A'). See color map, Figure
2.11,
for the line of the cross section.
and combined to create stereopairs of photos, in
which relief of the ground surface can easily be seen.
Aerial photos are valuable tools for use by earth sci-
entists in studying geologic and environmental phe-
nomena. Aerial photos have proven invaluable in
making topographic, geologic, and soil maps, in aid-
ing military operations, in regional planning, and in
monitoring the environment.
Federal, state, and other governmental and pri-
vate agencies have photographed practically all of the
United States and Canada from aircraft flying at alti-
tudes as great as 60,000 feet.
back to the plane. About 30,000 points per second are
recorded as the airplane flies over the land surface.
These data are then compiled into an image for inter-
pretation of natural and human-related features on
the ground. LIDAR is particularly powerful in terrain
that has tree cover, as the data allow "virtual defor-
estation" or removal of vegetation from the image.
Thus LIDAR images show the surface of the ground,
not the tops of the trees. Fault identification, landslide
recognition, floodplain delineation, and basic geo-
logic mapping are among the applications in environ-
mental geology. A LIDAR image analysis is included
in Exercise 6 where it is used to locate new faults that
had escaped traditional field and aerial photographic
techniques.
Satellite Imagery
Beginning in July 1972, with the launching of ERTS-1
(Earth Resources Technology Satellite), the United States
started collecting, on a repetitive basis, satellite images
of the earth's surface. Several similar Land Satellites
(Landsat) have subsequently been launched in near-
polar orbits at an altitude of about 570 miles to collect a
variety of images of large areas of the earth's surface.
Landsat images can be photographed in different wave-
lengths and may be produced in false color or enhanced
by computer to make available a tremendous amount of
data. The satellite passes over the same area at regular
intervals permitting monitoring of surface changes in
addition to providing information for mapping and
detecting changes in vegetation.
Many satellites from different national pro-
grams and private organizations are now providing
improved understanding of Earth processes, land-
forms, materials, and change in the Earth system.
Exceptional image resolution, availability on the
Web (e.g., Google Earth, TerraServer, etc.) and ease of
use are enticing the public to explore and under-
stand as never before how components of the Earth
work.
Interpretation of Photos and Images
The interpretation of either single aerial photos or
stereopairs requires practice. One of the greatest diffi-
culties experienced in viewing an air photo for the first
time is recognition of features on the photographs, such
as roads and buildings that are very familiar when
seen on the ground. However, someone unfamiliar
with aerial photos can comprehend them amazingly
well with a little practice and the guidance of a few
basic principles of recognition of common features on
black and white photos. Some aspects of features to
consider when looking at an aerial photograph are
listed in Table 2.2.
On natural color aerial photographs, colors are
similar to those seen on the ground. Some geological
studies use infrared photographs, in which healthy
vegetation appears as shades of red. Satellite images
are manipulated in computers to use colors to empha-
size different aspects of geology or vegetation. It is
important to pay close attention to color keys when
working with infrared photographs or satellite
images.
LIDAR
LIDAR (Light Detection And Ranging), also known as
Airborne Laser Scanning (ALS), is a relatively new
remote sensing technique that transmits laser light
from an airplane and records the light that bounces
Determination of Scale
The most commonly used aerial photographs are ver-
tical photos that accurately illustrate the land in the
