TheImprovement ofTropical and Subtropical Rangelands

88 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
array. Each picture element (pixel) equals an area on the ground of
approximately 0.5 hectares (60 m x 80 m). An image is created for
each band in the green, red, and two infrared parts of the spectrum.
Images from each band may be combined to create an image that
is similar in color renditions to a conventional CIR photograph, and
may be interpreted manually. Because they exist in digital form
as well, images may be proceaed statistically using a computer.
Although spatial resolution is relatively low, the MSS is well suited
to resource a.essment because a single image covers a large area and,
as suggested above, it is desirable sometimes to avoid the confusion
introduced by detailed data.
The most recent seri. of the National Oceanic and Atmospheric
Administration (NOAA) weather satellites has carried the
Advanced Very High Resolution Radiometer (AVHRR) instrument.
The AVHRR has low resolution (see table 4(1) because it was
intended to complement conventional very-low-resolution weather
satellite systems by acquiring data that could be used to deacribe
general land surface conditions. Although AVHRR has been used
mainly for monitoring studies, it might provide useful information
for exceptionally large regions.
Aerial Photography Aerial photographs are the most widely used
form of remote sensing data. They are routinely acquired in most
parts of the world for a variety of purposes, including geophysical
surveys and the production of topographic maps.
The principal advantages of aerial photography are its high quality
(conventional image format is about 23 cm x 23 cm) and the
ability to schedule photographic missions at the proper time and
appropriate scale using the desired films and filters. Moreover, aerial
photography firms can be contracted to fly missions over almost any
area in the world. The primary disadvantage of conventional aerial
photography is its relatively high cost.
To counter the high costs of conventional aerial photography,
increased use has been made of35 mmcameras for aerial photography
(figure 4-3a and 4-3b). Because the film format is so small, most 35
mm photography has been acquired at very large scales from lowflying
light aircraft. Images from these systems are ofsomewhat lower
quality than conventional aerial photography and they must be used
with some care. The advantages in cost and flexibility, however, seem
to offset most other considerations when the 35 mm system is used
for resource assessment.