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21. LIGHT SCATTERING MODELS AND REFLECTANCE
MEASUREMENTS IN REMOTE SENSING OF SNOW
The sensor registers the top of atmosphere radiance
for each
point of the image. This radiance consists of the components coming from
atmosphere, target and the surroundings of the target. To obtain the top of
terrain radiance,
the effects of the atmosphere have to be
corrected. Parameter refers to snow metamorphism because the reflectance
is different for different snow types (Table 1). This can be made by using
available software codes and for input the locations of Sun and sensor as
well as atmospheric parameters are needed. The main problem is the lack of
basic information for corrections (aerosols, humidity, pressure) which
usually is available only in places of permanent ground stations. Usually
average values of atmospheric characteristics must be used.
To calculate the incidence angle digital terrain model is needed in
addition to the locations of Sun and sensor. Incidence angle is the key
parameter in modeling the reflectance because BRDF is a function of this
angle. If the incidence angle and the BRDF are known it is possible to
calculate e.g., nadir radiances for all parts of the image. This allows the
comparisons between different images.
The BRDF is a function of the wavelength and in practice it is not
possible to measure the whole range of the spectrum. The BRDF can be
approximated by measuring only the most important parts of the spectrum
because the shape of the reflectance curve remains very similar even the
incidence angle changes (Figure 1).
3.2
Modeling of snow
The bidirectional reflectance distribution function contains a lot of
information and many measurements are needed for determining it. Thus it
is important to analyze the possible incidence angles before starting
measurements and modeling. For most of the snow covered areas in MarchMay in Finland the following ranges can be given:
This selection reduces the amount of measurements needed for
determining the BRDF of snow for practical applications.
21. LIGHT SCATTERING MODELS AND REFLECTANCE
MEASUREMENTS IN REMOTE SENSING OF SNOW
The sensor registers the top of atmosphere radiance
for each
point of the image. This radiance consists of the components coming from
atmosphere, target and the surroundings of the target. To obtain the top of
terrain radiance,
the effects of the atmosphere have to be
corrected. Parameter refers to snow metamorphism because the reflectance
is different for different snow types (Table 1). This can be made by using
available software codes and for input the locations of Sun and sensor as
well as atmospheric parameters are needed. The main problem is the lack of
basic information for corrections (aerosols, humidity, pressure) which
usually is available only in places of permanent ground stations. Usually
average values of atmospheric characteristics must be used.
To calculate the incidence angle digital terrain model is needed in
addition to the locations of Sun and sensor. Incidence angle is the key
parameter in modeling the reflectance because BRDF is a function of this
angle. If the incidence angle and the BRDF are known it is possible to
calculate e.g., nadir radiances for all parts of the image. This allows the
comparisons between different images.
The BRDF is a function of the wavelength and in practice it is not
possible to measure the whole range of the spectrum. The BRDF can be
approximated by measuring only the most important parts of the spectrum
because the shape of the reflectance curve remains very similar even the
incidence angle changes (Figure 1).
3.2
Modeling of snow
The bidirectional reflectance distribution function contains a lot of
information and many measurements are needed for determining it. Thus it
is important to analyze the possible incidence angles before starting
measurements and modeling. For most of the snow covered areas in MarchMay in Finland the following ranges can be given:
This selection reduces the amount of measurements needed for
determining the BRDF of snow for practical applications.