TheImprovement ofTropical and Subtropical Rangelands

92 IMPROVEMENT 0' TROPIOAL AND SUBTROPIOAL RANGELANDS
1. subclvlde lllllan-_Ie Im-ve
(e.g., Landllat) Into terrain regions
I
I
I
I
\, ,
\
2.
examine sample larger IlCllIeImages
(e.g., air photognpha)
3.
I \
I \
I \
I \
I \
YlsIt sample ground IocIIIItIea
within sample air photographs
FIGURE 4-5 The multiltage approach to lampling u applied in remote leBling
(after Townshend, 1981).
of information for mapping land resources. ~ previously noted,
however, existing aerial photography may be quite old and thus
of relatively little value for mapping current conditions. Even current
aerial photography, though, may present difficulties. Because
of the great amount of detailed information they contain, conventional
aerial photographs are commonly perceived to have considerable
intelligence value. ~ a result, their distribution is sometimes
controlled by military authorities and thus may not be available.
H the project is large enough, acquisition of aerial photography
may be a major activity and warrant special attention. A large number
of private firms provide aerial photography services. Their names
and addressses can be found in the telephone books of mrJor cities