TheImprovement ofTropical and Subtropical Rangelands

84 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
Remote Sensing
Although maps and estimates of land characteristics can be produced
by ground sampling alone, it is seldom done for a resource
assessment because of the expense and the likelihood of missing important
features.
Remote sensing is the most commonly used tool for gathering
information for large areas. .AIJ defined here, remote sensing includes
the uses of aerial photography and satellite imagery to study the
earth's surface. Remote sensing data are unique because they (1)
provide a comprehensive picture and permanent record of surface
conditions at one point in time and (2) present a vertical perspective
in which all features are represented, essentially in their true geometric
relationship with all other features. There is no ideal remote
sensing system. Thus, a primary task in remote sensing is to select a
system that best meets the needs of the project.
Principlea of Remote Senftn, Remote sensing exploits the difFerences
that can be detected among surface features on an image of the
earth. The ability to distinguish among features is conditioned by
several factors. Foremos~ in many applications is the feature's tone
or color. Earth materials reflec~ or emit electromagnetic radiation,
including light, in different ways (figure 4(2). For example, vegetation
has a unique pattern of reflectance, with moderate reflectance
in the green part of the spectrum, low reflectance in the red part
of the spectrum, and very high reflectance in the infrared part of
the spectrum that is just beyond what is visible to the human eye.
Second are ~hose inherent properties of a feature that determine how
it appears, or what is sometimes called a feature's "signature" or "response."
The characteristic shape of a surface feature when viewed
from above (for example, a folded geologic structure) or its relative
size (such as tree versus shrub) are two such properties. Other important
characteristics are less obvious, such as a feature's "texture"
on an image (say the difference between the smooth texture of a
meadow and the rough texture of a forest canopy), or the association
of one feature with others (such as pine forests on steep north-facing
mountain slopes).
The type ofremote sensing system used also affects the ability to
distinguish among earth features. Scale-the relationship between
the size of the image and the area on the ground it portrays-largely
determines w~at can be seen, especially if geometric properties such