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| | | | | | | | | Animation of two ASTER satellite images of the Kronebreen glacier taken on 26 June 2001 and 6 August 2001 | |
Multitemporal image correlation
By correlating two images of a glacier taken at different times it is possible to measure the displacement of the glacier between Time 1 and Time 2. This technique is called ‘image matching’ or ‘feature tracking’.
The principle of image matching consists in identifying the same point in two satellite images taken at different times. If, between Time 1 and Time 2, this point has moved, for instance due to glacier flow, the point will have different coordinates in both images. The difference in these coordinates is the horizontal movement of the point.
Unfortunately this technique doesn’t work for all glaciers and images. It is important that corresponding points be identifiable in both images. This will not be the case if the ice melt was too strong between the two acquisition times, or if a snow fall covered the glacier at one of the two times. In addition, large glacier parts might not have sufficient objects on their surface to track.
Displacement field of Kronebreen glacier
Computer programmes allow us to measure the displacement of corresponding points between two repeat images. This works not only for selected points but for a dense network of points over the entire image. This means that entire ice velocity fields can be measured from space.
| | ASTER image of Kronebreen glacier taken on 26 June 2001 | |
Questions
Print out the two images (26 June 2001 and 6 August 2001) that make up the previous animation. The image height corresponds to 4.3 km. Place a transparency over the first image.
| | | ASTER image of Kronebreen glacier taken on 6 August 2001 |
Copy some of the large crevasses and some distinct points outside the glacier. Then place the transparency on the second image and note which crevasses have moved since the first image was taken.
Measure the displacement with a ruler and convert the distance to metres by using the image height of 4.3 km as scale.
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| | Glacier ice flow IntroductionBackground The dynamics of glaciersExercises Worksheet introductionExercise 1: Correlation of repeated images (LEOWorks 3)Eduspace - Software LEOWorks 3Eduspace - Download ASTER.zipGoogleEarth file
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