Environmental Engineering Reference
In-Depth Information
Fig. 3.1 Divisions of EM
spectrum
10 6 m) is one spectral band. Satellite sensors
have been designed to measure responses within
particular spectral bands to enable the discrimi-
nation of the major Earth
as the spectral signature of the material. All
Earth
s surface features, including minerals,
vegetation, dry soil, sand, water and snow, have
unique (speci
'
s surface materials.
Researchers or scientists choose a particular
spectral band for data collection depending on
what they wish to examine. The design of
satellite sensors is based on the absorption
characteristics of Earth
'
c) spectral re
fl
ectance signatures.
ectance of clear water is generally
low. However, the re
The re
fl
ectance is maximum at the
blue end of the spectrum and decreases as
wavelength increases. Hence, water appears dark
bluish to the visible eye. Turbid water has some
sediment suspension that
fl
s surface materials across
all the measurable parts in the EM spectrum.
'
ec-
tance in the red end of the spectrum and would
be brownish in appearance. The re
increases the re
fl
ectance of
bare soil generally depends on its composition.
In the example shown, the re
fl
3.1.3 Spectral Signature
ectance increases
monotonically with increasing wavelength.
Hence, it should appear yellowish red to the eye.
Figure 3.2 shows
fl
When the solar radiation reaches the surface of
the Earth, some of the energy at speci
c wave-
lengths is absorbed and the rest of the energy is
re
ectance
spectra of water, bare soil and two types of
vegetation.
When solar radiation hits a target surface, it
may be transmitted, absorbed or reflected.
Different materials reflect and absorb differently
at different wavelengths. The re
the typical
re
fl
ected by the surface material. The only two
exceptions to this situation are whether the sur-
face of a body is a perfect re
fl
ector or a true black
body. The occurrence of such materials in the
planet Earth is extremely rare. In the visible
region of the EM spectrum, the feature we
describe as the colour of the object is the visible
light that is not absorbed (but re
fl
ectance spec-
trum of a material is a plot of the fraction of
radiation re
fl
ected) by that
object. In the case of a green leaf, for example,
the blue and red wavelengths are absorbed by the
leaf, while the green wavelength is reflected and
detected by our eyes.
fl
ected as a function of the incident
wavelength and serves as a unique signature for
the material.
fl
In principle, a material can be
identi
ed from its spectral re
fl
ectance signature if
In remote sensing, a
the sensing system has suf
cient spectral reso-
lution to distinguish its spectrum from those of
other materials. This premise provides the basis
for multispectral remote sensing.
Vegetation has a unique spectral signature that
enables it to be distinguished readily from other
types of land cover in an optical/near-infrared
image. The re
detector measures
the EM radiation that
is
re
s surface materials.
These measurements can help to distinguish the
type of land covering. Soil, water and vegetation
have clearly different patterns of re
fl
ected back from the Earth
'
ectance and
absorption over different wavelengths. The
re
fl
ectance of radiation from one type of surface
material, such as soil, varies over the range of
wavelengths in the EM spectrum. This is known
fl
ectance is low in both the blue
and red regions of the spectrum, due to absorp-
tion by chlorophyll for photosynthesis. It has a
fl
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