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0 
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greyvalue greyvalue 
  
Figure 3: Luminance values of an image at night with illumination (a) with yellow balls and at night without 
illumination (b). Greyvalue 0 corresponds to black, greyvalue 255 corresponds to white in each 
histogramm. It is obvious that at night without illumination it not necessary to work with colors. 
Contrast between bright balls and dark surrounding is clear enough to be determined by the computer 
(b). 
from the previous images and predicts the search strategy for the new one. 
ACKNOWLEDGEMENTS 
This work is supported by the Swiss National Fonds in the programme *Climate Change and natural hazards". 
Further thank is given to Walter Caviezel, Urs Suter and Angelo Maccagnan of the Swiss Federal Institute for 
Snow and Avalanche Research for their support. 
REFERENCES 
Beyer, H. (1990). Bestimmung von Insektenflugbahnen mit digitaler Photogrammetrie. Technical Report 170, 
Institute for Geodesy und Photogrammetry-Eidgenóssische Technische Hochschule Zürich. 
Kobayashi, D. (1987). Snow accumulation on a narrow board. Cold Regions and Technology, 13:239-245. 
Lundberg, A. and Halldin, S. (1994). Evaporation of intercepted snow: Analysis of governing factors. Water 
Resources Research, 30(9):2587-2598. 
Maas, H.-G. (1992). Ein photogrammetrisches System zur Bestimmung von Geschwindigkeitsfeldern in turbu- 
lenten Stromungen. Allgemeine Vermessungsnachrichten, 8-9:373-383. 
Schmidt, R. A. and Pomeroy, J. W. (1990). Bending of a conifer branch at subfreezing temperature. Can. J. 
For. Res., 20:1250-1253. 
Tennyson, L. C., Ffolliott, P. F., and Thorud, D. B. (1974). Use of time-lapse photography to assess potential 
interception in arizona ponderosa pine. Water Resources Bulletin, 10(6):1246-1234. 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995