Before describing the velocity field, a brief overview of measurement techniques may be helpful. In the early days of glaciology, velocity measurements were commonly made by triangulation from fixed points on stable surfaces off of the glacier. I have spent many hours peering through a theodolite at stakes drilled into a glacier. In the 1970s, electronic theodolites with laser distance-ranging capabilities greatly reduced the effort needed to make a measurement. Because the distance can be measured directly with such an instrument, a stake location can be determined by occupying only one fixed point rather than two, and electronic display of angles saves hours of adjusting a vernier. Later technological developments have resulted in computer-driven systems that can activate the theodolite and track the stakes. Thus, measurements can be made automatically as often as desired.
Velocity measurements far from stable fixed points are next to impossible with theodolite-based techniques. The advent of geographical positioning systems (GPS) technology solved that problem. GPS units measure the distance to a fleet of satellites with orbits that are known very accurately. With the most precise GPS units, the location of a point can be determined to within a couple of centimeters. Such units are now used to measure rates of continental drift.
An exciting development that is quickly leading to a much better understanding of flow fields in large ice sheets is the use of satellite images taken at, say, intervals of days to a year or so, to determine the velocity field. The images are coregistered based on the assumption that
Figure 5.1. Schematic diagram illustrating dependence of u on bn on an ice sheet.
1-► x certain features are stationary or moving only slowly compared with others. Then, cross-correlation techniques are used to compare crevasse patterns or other recognizable moving features on the two images and to determine how far they have moved between the dates of the respective images (Bindschadler and Scambos, 1991; Whillans and Tseng, 1995). High-speed digital computers are used to make these comparisons. In this way, a detailed quantitative map of the flow field can be produced. This technique, called satellite interferometry, is revolutionizing the measurement of velocities that, a scant 35 years ago, could be obtained only by tedious precision surveying with a theodolite.
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