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which always 
upwards and 
NOAA /VHRR 
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27. 
VI Supporting activities 
Interpretation of satellite data will always have to be accompanied by 
supporting activities, which can be divided in the following major groups: 
- ground observations and measurements 
- spectral properties of snow 
- underflights 
Ground truth is necessary for a better understanding of the study area and 
the influence caused by the various snowparameters on the registration of 
the snowcover by different sensor systems. For digital processing, applying 
supervised technology, the optimal selection of the test samples is of 
particular importance. Finally the verification of the classification results 
has to be secured. 
Underflights by aeroplanes or helicopters may serve similar purposes such as 
theverification of the results and selection of the test samples. 
The spectral characteristics of snow vary considerably, due to illumination, 
season, climatic region, surface conditions etc. Besides snow is always 
covering an underlying stratum which may influence the spectral reflectance, 
or is partly covered by vegetation, especially trees. These problems will not 
be treated in this paper. Sofar it may be concluded that for efficient snow 
studies data in the following three channels are absolutely essential: 
- one band in the visible spectrum (Landsat-5) to determine the areal extent; 
- one band in the near IR (Landsat-7) to separate dry and wet snow; 
- one band in the 1.55-1.75 pm (to be included in Landsat-D) to separate 
snow from clouds. 
The ultimate goal will be to reduce ground measurements and observations to 
a minimum, e.g. by building up data banks on the spectral behaviour of snow 
etc. 
VII Conclusions 
Methods for snowcover monitoring today may be regarded as quasi operational. 
For further projects it is especially important that the selected method is 
appropriate regarding the size, structure and climatic conditions of the 
study area, sensor system, accuracy needed to fulfill the specific task 
within a very short time lapse. The most promising approach offer interactive 
interpretation systems,where the knowledge of an experienced interpreter can 
be combined with fast digital classification systems.