USGS Geoscience Data Catalog
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Lucchitta, B.K., Barrett, J.M., Bowell, J.A., Ferrigno, J.G., Mullins, K.F., Rosanova, C.E., and R.S. Williams, Jr., 1995, Velocities of outlet glaciers, ice streams, and ice shelves, Antarctica, from satellite images: U.S. Geological Survey, Flagstaff, Arizona.Online Links:
This local coordinate system was used: We generally register Landsat 1, 2, and 3 images to Landsat 4 and 5 images, because the latter have more stable internal geometry and higher resolution than the earlier images. We digitally co-register the images by using a minimum of three well-dispersed fixed points (such as nunataks or ice walls) to calculate a least-squares fit to a first-order polynomial equation. This insures that only a rotational/ translational correction is made and no new internal error is introduced during the geometric resampling. In the interactive technique, we then match and align the crevasse patterns displaced with time, and record the starting/ending image coordinates for each point. To obtain the distribution of average velocities over the length of the glacier tongues, we also use the distance from the location of each point on the earlier image to a base line drawn perpendicular to glacier movement and ideally lying on the grounding line; where the grounding line is complex, the base line may only approximate its position. Next, a digitized file is made, tracing the glacier ice movements and defining the glacier's baseline ( or grounding line). This file is used to calculate the velocity and distance statistics by measuring the displacements along the curve that approximates the ices movement per given time interval. For each measured point, a displacement vector is plotted on the image, commonly the earlier one of the pair, to illustrate the relative velocities between glaciers and time intervals.
Although in principle the images could be registered to the earth's surface, for this exercise georeference is not necessary, since the objective is merely to understand ice movement through time and among paths within a glacier tongue. Hence the data are not explicitly georeferenced.
(520) 556-7176 (voice)
(520) 556-7014 (FAX)
blucchitta@iflag2.wr.usgs.gov
Changes in global climate and sea level are intricately linked to changes in the area and volume of polar ice sheets. Thus, melting of the ice sheets may severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic ice sheet alone could raise sea level by approximately 5 m. In spite of their importance, the current mass balances (the net gains or losses) of the Antarctic ice sheets are not known. Because of difficult logistic problems in Antarctica, field research has focused on only a few major ice streams and outlet glaciers. Yet, to understand the ice sheet dynamics fully, we must carefully document all of the coastal changes associated with advance and retreat of ice shelves, outlet glaciers, and ice streams.
A critical parameter of ice sheets is their velocity field, which, together with ice thickness, allows the determination of discharge rates. Remote sensing, using moderate- to high- resolution satellite images, permits glacier movement to be measured on sequential images covering the same area; the velocities can be measured quickly and relatively inexpensively by tracking crevasses or other patterns that move with the ice. Especially important are velocities where the ice crosses the glaciers grounding lines (locations along the coast where the ice is no longer ground supported and begins to float).
Ferrigno, J.G., Lucchitta, B.K., Mullins, K.F., Allison, A.L., Allen, R.J., and Gould, W.G., 1993, Velocity measurements and changes in position of Thwaites Glacier Ice Berg Tongue from aerial photographs, Landsat images, and NOAA AVHRR data: Annals of Glaciology v.17, p. 239-244.
Lucchitta, B.K., Mullins, K.F., Allison, A.L., and Ferrigno, J.G., 1993, Antarctic glacial tongue velocities from Landsat images: First results: Annals of Glaciology v. 17, p. 356-366.
Lucchitta, B.K., Smith, C.E., Bowell, J.A., and Mullins, K.F., 1994, Velocities and mass balance of Pine Island Glacier, West Antarctica, Derived from ERS-1 SAR images: Ecological Society of America Special Publication 361, p. 147-151.
Lucchitta, B.K., Mullins, K.F., Smith, C.E., and Ferrigno, J.G., 1994, Velocities of Smith Glacier Ice Tongue and Dotson Ice Shelf, Walgreen Coast, Marie Byrd Land, West Antarctica: Annals of Glaciology v. 20, p. 101-109.
Lucchitta, B.K., Smith, C.E., and Mullins, K.F., 1995, Velocities and mass balance of Pine Island Glacier, West Antarctica: Annals of Glaciology v. 21.
Data sources produced in this process:
Data sources produced in this process:
We use two methods to determine the glacial velocities: an interactive one in which we visually trace crevasse patterns (Lucchitta and others, 1993) and an autocorrelation program developed by Bindschadler and Scambos (1991) and Scambos and others (1992). First, we digitally co-register the images by using a minimum of three well-dispersed fixed points (such as nunataks or ice walls) to calculate a least-squares fit to a first-order polynomial equation. This insures that only a rotational/translational correction is made and no new internal error is introduced during the geometric resampling. In the interactive technique, we then match and align the crevasse patterns displaced with time, and record the starting/ ending image coordinates for each point. To obtain the distribution of average velocities over the length of the glacier tongues, we also use the distance from the location of each point on the earlier image to a base line drawn perpendicular to glacier movement and ideally lying on the grounding line; where the grounding line is complex, the base line may only approximate its position. Next, a digitized file is made, tracing the glacier ice movements and defining the glacier's baseline (or grounding line). This file is used to calculate the velocity and distance statistics by measuring the displacements along the curve that approximates the ices movement per given time interval. For each measured point, a displacement vector is plotted on the image, commonly the earlier one of the pair, to illustrate the relative velocities between glaciers and time intervals.
Accuracy of point positions is limited by the digital representation of the images. The accuracy with which individual features of the ice tongues are correlated (from image to image) cannot be assessed, because it is confounded with the spatial variation of the velocity field.
The files contained in this data base are the output ASCII files
generated by this statistical software. Each file identifies the
images used, their dates, and resolutions, the time interval
between image acquisitions and the statistical variables used to
make the calculations. These data are followed by a table of the
distance and velocity values for each point and the statistics
calculated per path. The measurement results are shown in graphs
that display average velocities per given time interval versus the
distance from the base line for all points in each field (not
included in this data base).
In the auto-correlation method we use the same techniques for
coregistration and graphic and statistical display. However, we
may not divide the glaciers into segments and paths, but instead
combine all velocities and show variations across the glacier by
color contours (also not shown in this report).
Because the velocity field may also change across the glacier tongues, we divide the wider glaciers into several longitudinal paths. Next we obtain an estimate of the spread of measured points by performing a regression analysis on the data. This includes an option to cull bad data points by inputting a variable for the standard deviation. If used, the mean absolute deviation of the points about this line is calculated and any points lying outside that distance are disregarded during the statistical analysis. Calculations are made for the entire glacier as well as for each individual path. The 95% confidence interval for the regression coefficient is calculated along with the correlation coefficient.
Are there legal restrictions on access or use of the data?
- Access_Constraints: none
- Use_Constraints: none
(703) 648-6533 (voice)
(703) 648-6560 (FAX)
pschweitzer@usgs.gov
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
| Data format: | Glacier-velocity data in format TEXT somewhat regular but not entirely tabular information |
|---|---|
| Network links: |
<http://geochange.er.usgs.gov/pub/antarctica/glacier-velocity/Core/raw/> |
(703) 648-6533 (voice)
(703) 648-6560 (FAX)
pschweitzer@usgs.gov
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://geo-nsdi.er.usgs.gov/metadata/other/glaciers/metadata.faq.html
Page Contact Information: Peter Schweitzer
Page Last Modified: Friday, 19-Oct-2007 13:37:03 EDT
