Document Type

Article

Publication Date

1979

Abstract

Processes typical of arid environments can be monitored from space by comparing orbital photographs taken over a period of years. Using the zoom transfer scope, it is possible to superim- pose two photographs and detect changes occur- ring with time. Oblique photographs can be used, but best results are obtained with vertical or near- vertical photographs. Changes observed by comparing t the 1975 Apollo-Soyuz photographs with data from pre- vious missions include increase vegeta- tion west of the Nile Delta of approximately 1108 km? in 10 years; (2) an average shift of 2.5 km over a 6-year period in the sand patterns of the Oweinat Mountain region at the borders between Egypt, Libya, and Sudan, and (3) F reduction in the water level of Lake Chad and dune encroach- ment upon the lake over a period of 9 years. As a result of this investigation, it is concluded that studies of temporal changes in arid and other regions are facilitated when orbital photographs are vertical and are taken from similar altitudes.sible in the field, Processes characteristic of arid enyironments are examined in this paper by com- paring photographs taken during the Gemini, Apollo, Skylab, and Apollo-Soyuz missions. The ihree processes considcred in this paper in- clude the following: (l) land reclamation, specifically on the western margin of the Nile N River Delta in northern Egypt, (2) shifts in eolian sand patterns, as seen in the Oweinat Mountan : region at the borders betwecn Egypt, Libya, and Sudan; and (3) changes in lake boundaries, as in the example of Lake Chad in north-central Africa (is. 1). The comparison of photographs was facilitated by the use of the model ZT-4 zoom transfer scope (Bausch and Lomb), This instrument enables the projection of one photograph onto another, despile differences in obliquity and scale, by PP tically rolating, stretching, and enlarging the photographs. Only when they are vertical or near- vertical can entire photographs be overlaid, When using เwo Dhotowraphs of varying seale and obllq- uity, it is nccessary 0) continual ly adiust the photographs from arca to arcs. By superimposing permanent Teatures, possible to compare the area |surrounding cach feature by transpos detaits from one photograph onto another. he best technique when using oblique photograph: to fix three points, which form a tniangle. This procedure will ensure that the transposition ts.cor- rect more than hone transter scope can also be used to Ch, Che z0om make acet I, mewsurements by transla direeily onto maps.

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