Environmental Engineering Reference
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Fig. 3.2 Typical
reflectance spectra
peak at the green region. In the NIR region, the
re
re
ectance spectrum also depends on other fac-
tors such as the leaf moisture content and health
of the plants. These properties enable vegetation
condition to be monitored using remotely sensed
images.
fl
ectance is much higher than that in the visible
band due to the cellular structure in the leaves.
Hence, vegetation can be identi
fl
ed by the high
NIR but generally low visible re
ectance. This
property has been used in early reconnaissance
missions during war
fl
times
for
'
camou
fl
age
'
3.1.4 Sensors and Platforms
detection
.
The shape of the re
fl
ectance spectrum can be
used for
identication of vegetation
type
A sensor is a device that measures and records
EM energy. Sensors can be divided into two
groups (Fig. 3.4 ). Passive sensors depend on an
external source of energy, usually the sun. The
most common passive sensor is the photographic
camera. Active sensors have their own source of
energy, e.g., a radar gun. These sensors send out
a signal and measure the amount re
(Fig. 3.3 ). For example, the re
ectance spectra of
dry grass and green grass in the previous
fl
gures
can be distinguished although they exhibit the
general characteristics of high NIR but low vis-
ible re
fl
ectance. Dry grass has higher re
fl
ectance
in the visible region, but lower re
ectance in the
NIR region. For the same vegetation type, the
fl
fl
ected back.
Fig. 3.3 Typical
reflectance spectra of
vegetation
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