Snow line mapping using radar imagery, Place Glacier, B.C.
Date
1995
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Degree Level
Masters
Abstract
This thesis evaluates the effectiveness of ERS-1 synthetic aperture radar (SAR) imagery for mapping movement of the transient snow line in the Place Glacier
basin during an ablation season. Hydrological models of mountainous regions require
frequent and accurate measurement of snow and ice covered areas to produce timely
forecasts. Studies of glacier mass balance also require this information to monitor
glacier health and assess changes in water resources. These areas are extensive and
often inaccessible. Snow and ice accumulation can be effectively mapped using
imagery from optical sensors such as Landsat-TM. However, the presence of cloud
cover often limits the use of optical image data at times when it is required. SAR data
can be obtained independent of weather and time of day, thus providing a potentially
timely source of hydrological and glaciological data.
The two primary objectives of this study are to normalise the topographically
induced distortions (radiometric and geometric) inherent in SAR imagery of rugged
terrain and to delineate the snow line in the normalised imagery as the boundary
between the wet snow and glacier ice facies. SAR images acquired on June 19, 1992,
August 28, 1992, and October 2, 1992 were chosen to represent different stages of the
ablation period. A digital elevation model (DEM) with 60 m grid spacing is created
from survey and digitised map data to provide the topographic information necessary
for normalising the SAR imagery. The radiometric distortions are normalised with a
cosine correction and the image texture is enhanced to take advantage of the spatial distribution of tonal variations within each image. To minimise geometric distortions
and georeference the imagery each cosine corrected SAR image is ortho-rectified to
an error of 40 m or better using the DEM and satellite orbital and ephemeris data. A
supervised classification is performed on the ortho-rectified imagery to map the spatial
distribution of snow and glacial ice within the basin.
In June, when the glacier is still snow covered, the normalised imagery shows a
significant difference between the return for wet snow at low incidence angles ( < 30°)
and that for wet snow at large angles(> 30°), the former being several times greater.
This is probably due to the difference between surface scattering at small incidence
angles and volume scattering at large angles.
The visual boundary between the wet snow and glacier ice surfaces on the
ortho-rectified and classified August and October images is within 100 m horizontally
and 50 m vertically of the snow line vectors obtained from field data. The glacier
boundary is also discernible to within 50 m of the 1994 glacier outline. Several
isolated bare ice areas that are marked with crevasses or runoff runnels give a low
return similar to wet snow resulting in some confusion between glacier ice and wet
snow.
Despite the localised confusion between glacier ice and wet snow, examination
of the methodology shows that SAR imagery is an effective and relatively inexpensive
means of mapping glacier surface types.
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Degree
Master of Science (M.Sc.)
Department
Geography
Program
Geography