The Ultimate Field Trip (Part 4 of 8)

An Astronaut's View of Earth

by Kathryn D. Sullivan, Ph.D.
NASA Astronaut


RUBBER-SHEETING


Great Salt Lake, 1973
[SLS-40-201 processed, S87-49187]

Great Salt Lake, 1985
[STS51B-146-122 processed, S87-49188]


Great Salt Lake Composite, 1973-1985 [S88-26634]
This composite of the rubber-sheeted photos shows that the Great Salt Lake swelled from the white area measured from Skylab in 1973 to the encompassing yellow area photographed from STS-41G in the fall of 1984, plus the red areas added less than a year later, in the spring of 1985, photographed from STS-51B. The same process can be used to measure the shrinkage of a lake in drought, such as Lake Chad between 1966 and 1990.

As as example of how such photographic images from space can play a valuable role in scientific studies, consider this demonstration project. Astronaut photographs taken of the Great Salt Lake in 1973, 1984, and 1985 were digitized to measure changes in lake area. Some of the photos were oblique views, so the first processing step involved adjusting them to the nadir view one normally uses in mapping. Key features at and near the shoreline were identified on both the image and the published topographic map, so that the computer could match them. In this process, the picture is treated by the computer like a rubber sheet that is stretched or warped as needed to bring the photo points into coincidence with the same points on the map. The rectified photos then all match geometrically, so differences between them can be identified by calculating the change in brightness value measured at each picture element (pixel) from one year to another. (Data from satellites or aerial surveys can be merged or compared with the astronaut imagery in the same way, if desired.)