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Methodological aspects and regional examples of snow cover mapping from ERTS-1 and EREP imagery of the Swiss Alps

H. Haefner and K. Seidel
Proceedings Symposium of Frascati


A 'quick-look' method of mapping snow cover and measuring the areal extent of its surface has been developed, allowing rapid interpretation of each image. The method combines the advantages of an automatic photo-quantiser (Quantimet QTM 720) with high-resolution photographic techniques for a rapid determination of the variation in the snow cover and its boundaries. In a first step, the critical density level for each image between snow-and-ice-covered and uncovered land is discriminated individually on the QTM monitor by visual control. Since the whole image can be examined on the screen, it is possible to adjust the critical tone level to a median and generally accept- able position. In a second step, a photographic technique is used for the density slicing since the QTM, with a resolu- tion of 720 lines, does not provide a sufficiently detailed analysis of an entire ERTS picture. The calibrated position of the discriminated snow/ no-snow boundary in the Quantimet is used as a reference time to expose a high-resolution litho- graphic film , which records the separated snow cover in all detail. In a third step, the areal measurement is carried out, again with the QTM: the total snow cover is determined by measuring quarter sections of an image each time, to achieve the necessary resolution. By applying masking techniques, regions of any size, form and function (e.g. watersheds, administrative regions, etc.) can be measured. In a last step, the separated snow cover is trans- ferred onto a map (1: 500 000) on which the altitudi- nal positions of the temporary snow lines can be established for each location. The method is demonstrated for a test area of 2 about 170km in the southeastern Swiss Alps (Bergell/Grisons) and with vertical differences of about 2000 m. The results are combined and com- pared with ground information. The method could also be applied to data from a source other than ERTS.

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  author = {H. Haefner and K. Seidel},
  title = {Methodological aspects and regional examples of snow cover mapping from ERTS-1 and EREP imagery of the Swiss Alps},
  booktitle = {Proceedings Symposium of Frascati},
  year = {1974},
  pages = {155--165},
  volume = {ESRO SP-100},
  keywords = {}