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: Proceedings, 2nd ERS-1 Symposium, Hamburg, Germany, 11-14 Oct. 1993, ESA SP-361, p. 147-151.
Lucchitta, B.K., Mullins, K.F., Smith, C.E., and Ferrigno, J.G., in press, Velocities of Smith Glacier Ice Tongue and Dotson Ice Shelf, Walgreen Coast, Marie Byrd Land, West Antarctica: Annals of Glaciology, v. 20.
Lucchitta, B.K., Smith, C.E., and Mullins, K.F., Velocities and mass balance of Pine Island Glacier, West Antarctica: Submitted to Annals of Glaciology.
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).
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).
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. Several tests were made to compare the internal geometry of 3rd and 4th generations negatives with the original digital data. All of these tests, as well as several made between original and scanned images of transparencies, showed an insignificant degree of geometric error between products. These tests demonstrate that geometrical errors within the transparencies will contribute little to statistical variance between measurements. Loss of resolution and misidentification of features play a more important role in measurement error made with these images. Borgeson and others (1985) found that Landsat 5 images are accurate to about 0.4 pixels, meeting national Horizontal Map Accuracy standards for scales of 1:100,000 and smaller, and that Landsat 4 images are accurate to 0.8 pixel levels. Welch and others (1985) reported that Landsat 4 and 5 images meet accuracy standards for maps of 1:50,000 scale or smaller and are well suited to maps of 1:100,000 scale.
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.