Contents of this page
Photo: Martin Hoelzle
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Contents of this page
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In addition to the yearly measurements of horizontal and vertical deformation at two boreholes (1 and 2 in Figures below), photogrammetric investigations were performed at Pontresina Schafberg using techniques of analytical aerophotogrammetry. Changes in surface elevation over the period of 1971 to 1991 have been determined by comparing two digital terrain models (DTM) having a regular grid width of 25m (see Figure below). Most measured changes are not significant, because they are lower than the accuracy of the elevation measurements (±2 cm/a). Larger surface lifting observed in the northern part of the site is assumed to be caused by scree accumulation from the rock wall delimiting the cirque in northern direction. Surface lowering in the northwest of the site below borehole 2 and just above the steep channel running to Pontresina may indicate surface erosion.
Surface velocities between 1971 and 1991 were determined using
a special photogrammetric technique by simultaneously comparing
aerial photographs taken from different places as well as taken
at different times (see Figure below). Partially, the measured surface movements
of up to 10cm/a can only be interpreted qualitatively, for their
error range is around ±2-3 cm per year and, therefore, some results
are marginally significant. However, a comparison between the
photogrammetric results and the measurements in the boreholes
(4cm/a at borehole 1, ca. 1cm/a at borehole 2) shows quite good
agreement. The coherent surface velocity field of the two rock
glaciers indicates stress coupling and, thus, points to a connected
ice content of the frozen ground. Calculation of flow lines assuming
a time-constant flow field allows for an age estimate of the rock
glacier surface up to several thousand years.
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