1 October 2003
d. The measurement is best taken from one end of the photo to the other, passing
through the center (because error in the image increases away from the focus point). For
precision, it is best to average a number of ratios from across the image.
e. Photos are interpreted by recognizing various elements in a photo by the distinction
of tone, texture, size, shape, pattern, shadow, site, and association. For instance, airport
landing strips can look like roads, but their large widths, multiple intersections at small
angles, and the positioning of airport hangers and other buildings allow the interpreter to
correctly identify these "roads" as a special use area.
f. Aerial-photos are shot in a sequence with 60% overlap; this creates a stereo view
when two photos are viewed simultaneously. Stereoscopic viewing geometrically corrects
photos by eliminating errors attributable to camera tilt and terrain relief. Images are most
easily seen in stereo by viewing them through a stereoscope. With practice it is possible
to see in stereo without the stereoscope. This view will produce a three-dimensional im-
age, allowing you to see topographic relief and resistant vs. recessive rock types.
g. To maintain accuracy it is important to correlate objects seen in the image with the
actual object in the field. This verification is known as ground truth. Without ground truth
you may not be able to differentiate two similarly toned objects. For instance, two very
different but recessive geologic units could be mistakenly grouped together. Ground truth
will also establish the level of accuracy that can be attributed to the maps created based
solely on photo interpretations.
h. For information on aerial photograph acquisition, see Chapter 4. Chapter 5 presents
a discussion on the digital display and use of aerial photos in image processing.
2-9 Brief History of Remote Sensing. Remote sensing technologies have been
built upon by the work of researchers from a variety of disciplines. One must look further
than 100 years ago to understand the foundations of this technology. For a timeline his-
The chronology shows that remote sensing has matured rapidly since the 1970s. This ad-
vancement has been driven by both the military and commercial sectors in an effort to
effectively model and monitor Earth processes. For brevity, this overview focuses on
camera use in remote sensing followed by the development of two NASA programs and
France's SPOT system. To learn more about the development of remote sensing and de-
a. The Camera. The concept of imaging the Earth's surface has its roots in the devel-
opment of the camera, a black box housing light sensitive film. A small aperture allows
light reflected from objects to travel into the black box. The light then "exposes" film,
positioned in the interior, by activating a chemical emulsion on the film surface. After
exposure, the film negative (bright and dark are reversed) can be used to produce a posi-
tive print or a visual image of a scene.