the collection of elevation data along many parts of the
transects, leaving many gaps in the elevation data. Those areas
for which estimates could be made are indicated with velocity
vectors. In these areas, details of the flow character are clear,
but the failure to retrieve velocities over part of the eastern
flight line, and nearly all of the western flight line demonstrate
one of the limitations of using laser altimetry for velocity
estimates — the weather.
Results on the northeastern line at the 65°N ice margin show
roughly parallel flow along the transect with some local areas
of varied direction. In the southern portion of that same line,
velocities are almost 1.4 km/yr. The southwestern flight line
shows well-developed flow emptying into the basin with a
maximum velocity of nearly 1.2 km/yr. Such flow rates are
unusual, given the fact that the ice in this region appears to
have the characteristics that are more like an ice sheet margin
rather than an outlet glacier.
The results of the ATM-derived velocity estimate indicate that
for sufficiently rough surfaces, detailed velocities can be
retrieved along essentially a straight line. While such an
application is not very useful for large-scale ice sheet studies, it
is quite useful for detailed glacier studies (either for outlet
glaciers in Greenland, or for smaller mountain glaciers).
Because the elevation characteristics are accurately measured
with the ATM, a three-dimensional velocity field can be
retrieved. Moreover, when combined with thickness data,
generally collected coincidentally using an airborne ice
penetrating radar (Chuah, 1996), velocities derived from cross-
glacier passes allows for the calculation of ice fluxes.
A series of passes up an down a glacier would provide a
complete description of velocity fields, and subsequently the
full strain-rate tensor could be derived. In this way, the surface
flow mechanics of an entire glacier could be quickly
characterized. To make similar observations manually would
be extremely difficult, if not impossible. For practical purposes,
however, such an application of the ATM would require that
the instrument be mounted on a more maneuverable aircraft
than the P-3 that is currently used for the Greenland surveys.
The capability of conducting topography surveys from a more
agile Twin Otter platform using the ATM has been clearly
demonstrated through a series of beach-mapping missions
along most of the United States' east, west, and Gulf coasts.
Thus the application of the ATM for detailed surveys of
smaller glaciers is quite feasible.
5 SUMMARY
Building on the success that the Arctic Ice Mapping Program
has achieved for Greenland, the prospect for obtaining
important information on the state of balance of the Canadian
ice caps is quite good. Their smaller mass, in contrast to that
of the Greenland ice sheet makes them more likely to respond
to climate changes. A combination of these observations with
the in situ data that the Canadian researchers are providing,
will also allow an assessment of the significance of the
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
observations, and the relative contributions of ablation and
accumulation to the ice cap mass balance. By quantifying their
current mass balance, a clearer picture of the large-scale
response of the Arctic to the changing climate will be obtained.
The results from Canada are expected to provide useful insight
to the results obtained for Greenland, and those from
Greenland should be useful for the interpretation of the
Canadian data as well.
For more detailed studies of smaller glaciers, the ATM
observations show considerable potential, with the capability of
velocity determination based on the tracking of the movement
of elevation features. It is this measurement of detailed
elevation characteristics, as opposed to just visible features, that
allows a three-dimensional velocity field to be retrieved. For
future applications, a series of passes up an down a glacier
would provide a complete description of velocity fields and
subsequently the full strain-rate tensor. In this way, the surface
flow mechanics of an entire glacier could be quickly
characterized. The success of the ATM beach mapping
missions on a Twin Otter platform suggest that such a program
for detailed glacier studies is quite feasible.
6 REFERENCES
Abdalati, W. and W.B. Krabill, 1999. Calculation of ice
Velocities in the Jakobshavn Isbrae area using airborne laser
altimetry. [International Journal of Remote Sensing. 67, 194-
204.
Chuah, T.S., 1997. Design and development of a coherent
radar depth sounder for measurement of Greenland ice sheet
thickness. RSL Technical Report 10470-5. The University of
Kansas Remote Sensing Laboratory, Lawrence, KS. U.S.A.
Krabill, W.B., R.H. Thomas, C.F. Martin, R.N. Swift, and E.B.
Frederick, 1995. Accuracy of airborne laser altimetry over the
Greenland ice sheet. International Journal of Remote Sensing.
16(7), 1211-1222.
Krabill, W.B., E. Frederick, S. Manizade, C. Martin, J.
Sonntag, R. Swift, R. Thomas, W. Wright, and J. Yungel, 1999.
Rapid thinning of parts of the southern Greenland ice sheet.
Science, 283, 1522-1524.
Cogley, J.G., W.P. Adams, M.A. Ecclestone, F. Jung-
Rothenhausler, and C.S.L. Ommanney, 1996. Mass balance of
White Glacier, Axel Heiberg Island, N.W.T., Canada 1960-
1991. Journal of Glaciology, 44, 315-325.
Scambos, T.A., M.J. Dutkiewicz, J.C. Wilson, and R.A.
Bindschadler, 1992. Applications of image cross-correlation to
the measurement of glacier velocity using satellite data.
Remote Sensing of Environment, 42, 177-186.
Van der Veen, C.J., 1993. Interpretation of short-term ice sheet
elevation changes inferred from satellite altimetry. Climate
Change, 23, 383-405
Internation.
AIRBC
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