In the study of the sea bottom, considerable applica- 

 tion is being made of stereophotography, for which vertical 

 camera axes are best. Microrelief viewed from directly 

 above is in true plan view, but side detail is obscured. 

 Conversely, when viewed from the side, the relief is clear 

 but the plan view is highly distorted. For this reason 

 oblique photography is used mainly for rapid exploratory 

 work when side views of low relief features are desired 

 and measurements are of secondary importance. When 

 measurements of vertical height are the primary objective, 

 modern methods of photogrammetry are being widely used. 



Methods Used in the Interpretation of 

 the Photographs 



The vastness of the oceans prohibits complete 

 photographic and sediment sampling of the bottom surface 

 with present-day oceanographic equipment. We can, how- 

 ever, build up composite pictures of local and regional 

 bottom environments from collections of isolated camera 

 samplings and associated information, such as sediment 

 grain size, bottom currents, composition, nearness to 

 land, topographic barriers, and availability of food supplies 

 to benthonic organisms. 



A complication in the study and interpretation of 

 microrelief is that its structure is far from permanent. As 

 Crozier has pointed out, 5 very considerable sediment turn- 

 over goes on over relatively short periods of time. In the 

 shallow waters around Bermuda, in an area of 1 . 7 square 

 miles, he found that 500 to 1000 tons of sand passed through 

 holothurians (sea cucumbers) annually. This represents a 

 layer of sediment about 1 centimeter thick every 100 years, 

 and sediments have been forming for millions of years. 



To overcome the difficulties in deriving patterns of 

 origin and change on the sea floor, investigators use many 

 and varied techniques of photo interpretation. Some of the 

 more important are discussed in this section. 



Photographic contrast has been found to be an effec- 

 tive means of detecting and describing visible targets, and 

 of relating them to one another. Sharpness of definition is 

 required in aerial photographs, but is even more important 



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