203
21. LIGHT SCATTERING MODELS AND REFLECTANCE
MEASUREMENTS IN REMOTE SENSING OF SNOW
snow/terrain - sensor system. The reasons of these problems can be listed as
follows:
The reflectance of snow depends on the grain size and the presence of
liquid water in snow. These parameters varies due to the metamorphism
of snow which mainly is dependent on the age and temperature of snow.
The reflectance of snow depends on the incidence angle of measurement
and thus topography, Sun position, and sensor location affect on results
of measurements.
The reflectance of snow is different in different parts of the
electromagnetic spectrum and thus the channels of the sensor are very
relevant in snow cover monitoring.
The spatial resolution of sensors causes that in many cases the sensor
measures the reflectance of an area which only partly is snow covered.
This so called transition zone should be interpreted correctly, too.
Vegetation affects the reflectances of the terrain because it usually is not
covered by snow.
Due to the cloud cover, the repeat cycles of satellites and different swaths
of sensors it is in practice not possible to get images having a similar
imaging geometry. Thus to obtain useful results we have first to be able to
correct the effects of the incidence angle on reflectances, and secondly to
correct the effect of snow metamorphism on reflectances. After this it is
possible to model the reflectance of different terrain types covered partly or
totally by snow.
3.
SNOW COVER MODELLING
3.1
General
It is very important to know the angular dependence of snow reflectance
when remote sensing is used for snow characteristics determination. What
actually is needed is the bidirectional reflectance distribution function
(BRDF) because many sensors used in the visible and infrared parts of the
spectrum have wide swaths which means varying incidence angles for
radiance measurements. Wiscombe and Warren (1980) as well as Wald
(1994) have presented results concerning the angular dependence of snow
reflectance. Figure 1 presents the results. We see that the reflectance
increases with increasing incidence angle. The largest absolute variations
happen in the region of 0.9–1.4
This is a very important piece of
information for satellite image interpretation because the effect of varying
21. LIGHT SCATTERING MODELS AND REFLECTANCE
MEASUREMENTS IN REMOTE SENSING OF SNOW
snow/terrain - sensor system. The reasons of these problems can be listed as
follows:
The reflectance of snow depends on the grain size and the presence of
liquid water in snow. These parameters varies due to the metamorphism
of snow which mainly is dependent on the age and temperature of snow.
The reflectance of snow depends on the incidence angle of measurement
and thus topography, Sun position, and sensor location affect on results
of measurements.
The reflectance of snow is different in different parts of the
electromagnetic spectrum and thus the channels of the sensor are very
relevant in snow cover monitoring.
The spatial resolution of sensors causes that in many cases the sensor
measures the reflectance of an area which only partly is snow covered.
This so called transition zone should be interpreted correctly, too.
Vegetation affects the reflectances of the terrain because it usually is not
covered by snow.
Due to the cloud cover, the repeat cycles of satellites and different swaths
of sensors it is in practice not possible to get images having a similar
imaging geometry. Thus to obtain useful results we have first to be able to
correct the effects of the incidence angle on reflectances, and secondly to
correct the effect of snow metamorphism on reflectances. After this it is
possible to model the reflectance of different terrain types covered partly or
totally by snow.
3.
SNOW COVER MODELLING
3.1
General
It is very important to know the angular dependence of snow reflectance
when remote sensing is used for snow characteristics determination. What
actually is needed is the bidirectional reflectance distribution function
(BRDF) because many sensors used in the visible and infrared parts of the
spectrum have wide swaths which means varying incidence angles for
radiance measurements. Wiscombe and Warren (1980) as well as Wald
(1994) have presented results concerning the angular dependence of snow
reflectance. Figure 1 presents the results. We see that the reflectance
increases with increasing incidence angle. The largest absolute variations
happen in the region of 0.9–1.4
This is a very important piece of
information for satellite image interpretation because the effect of varying