TheImprovement ofTropical and Subtropical Rangelands

86 IMPROVEMENT OF TROPICAL AND SUBTROPICAL RANGELANDS
a "multispectral" approach by examining several different parts of
the spectrum. The value of this approach is easily appreciated when
comparing a black-and-white panchromatic photograph with a color
photograph. Color photography provides much more information
than black-and-white, but it tends to have poorer spatial resolution,
is more expensive, and is difficult to process in some parts of the
world because of the lack of proper equipment.
There is much information outside the visible spectrum that
would be useful for studying vegetation (table 4(1). Photographic
films have been developed that are sensitive to infrared radiation
that is just beyond the visible part of the spectrum. Color infrared
(Cm) film is the most common type. To record infrared energy
using conventional photographic technology, colors from the natural
environment are assigned to other colors on the cm film. Thus, the
final product is sometimes called a false color image: blue is filtered
out, green is recorded as blue, red is recorded as green, and infrared
is recorded as red. Because plants reflect more infrared light than
green light, green vegetation appears as various shades of red or
pink. Red soils are yellowish-green, and urban areas are bluish-gray.
cm photography is especially effective for mapping vegetation, but
is expensive and sometimes difficult to expose and process.
Nonphotographic sensing systems can be carried by aircraft and
spacecraft and provide similar kinds of spectral information. Nonphotographic
systems have a number of advantages. For example,
parts of the spectrum that are critical in some applications and
that are beyond the capability of photographic systems, such as the
thermal and microwave (radar), can be sampled. Also, many nonphotographic
systems record images digitally, which allows several
processing options (see below). With the exception of Landsat satellite
data, however, nonphotographic imaging systems will be used in
few resource assessments because of the expense of processing and
the need for special computer facilities.
Satellite Systems Since 1972, satellite imagery suitable for land resource
assessments has been produced continuously for most parts of
the world. Landsat was the first satellite to provide regular and universal
image data and continues to be the most widely used system.
The primary instrument on Landsat is the Multispectral Scanner
(MSS), which records images of the earth in four spectral bands (see
table 4(1). Images are recorded digitally but are produced in both
digital and photographic formats. The MSS creates an image by
recording the relative brightness of each element or cell of a large