The selection of an appropriate data acquisition system is of specific 
importance when developing an operational interpretation system which always 
has to be in a realistic relation to 
- the size, structure and climatic situation of the study area 
- the expected accuracy 
- the maximum processing time available 
- the financial funds available 
Landsat-data may be already used in small areas from 10 sq.km on upwards and 
are best suited for study areas of hundreds of sq.km, whilst for NOAA/VHRR 
data the area should exceed 1,000 sq.km. These relations between size of 
Study area, accuracy, sensor system in respect to ground resolution, repe- 
tition rate etc. are summarized in Fig. 2. Interesting possibilities could 
occur by combining two different data acquisition systems, e.g. a high 
resolution system with relative low repetation rate (Landsat) together with 
a System of high repetivity and low ground resolution (NOAA/VHRR). 
In an operational system there always will be a tradeoff between accuracy and 
time. Here the most important aspect has to be the time to deliver the vital 
informations to the user. In snowmapping no delay in transference of the 
classification results can be accepted and consequently the selected inter- 
pretation method has to fulfill this demand first of all. 
Fig. 3 systematically summarizes the different steps which have to be taken 
into account by the different methodological approaches to snowmapping. From 
a literature survey the following major methods could be extracted (Fig. 
4 - 6), grouped according to: 
- analog methods: 
photointerpretation 
optical devices 
electronical devices 
photographic devices 
- digital methods: 
- supervised systems 
- unsupervised systems 
- hybrid methods: 
- interactive systems 
Emphasis is given to the processingof mul ti temporal data. This 
causes no specific problems with the mentioned analog methods. But numerical 
processing requires special analysis of the data including geometric as well 
as radiometric corrections to make the results directly comparable. 
Finally a critical evaluation on the advantages and disadvantages of the 
most important methodological approaches are comprehended in Fig. 7. 
  
   
   
   
   
    
   
   
    
   
  
     
    
    
   
   
    
    
   
     
14. FT Mh ad ect ez ITO v0 c + ©