Proceedings PaperMosaic And Strip Scanning Photogrammetry Of Large Areas Underwater Regardless Of Transparency Limitations
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As is well known, no break-through has yet been achieved to improve underwater visibility to the point where very large areas could be seen or photographed in one shot, as common in aerial or space photography. An effective solution to this problem is simply to scan the bottom in parallel adjacent strips, using a specially designed vehicle and high capacity camera system. It will be also shown why precision mosaic photogrammetry is now made possible by the use of new water refraction correcting lenses. The necessity of particle back-scatter control by proper light placement and possible nanosecond light pulses and shutters will be shown as an essential factor in improving close-range visibility specially in turbid water. It will be shown why the camera-carrying vehicle must be designed especially for maximum attitude stability to achieve a straight track, constant altitude and constant speed controlled by a special instrument panel and control subsystems. Typical applications are in the fields of undersea cable, pipeline and fixed installation surveys, stereophotogrammetric mapping of the bottom, photography of large submarines and marine life, archeological surveys, landing beach and approach surveys. A recommendation will be made for replacing the old Secchi disk and transmissometer instruments by a new black target for visibility measurement, 1, THE SCANNING PRINCIPLE: Exactly as a single spot of modulated intensity, light travelling along parallel lines upon a phosphor screen will produce a meaningful picture on the total area of the screen, a large number of small area photographs taken at regular adjacent intervals along parallel lines on the bottom of the sea will yield a precise and detailed photographic mosaic when the individual pictures are assembled together along a proportionally scaled-down pattern. However, certain rules must be followed if the cost in time and money is to be held within reasonable limits. First, it is not acceptable to snap thousands of uncontrolled photos along random curved tracks, the result of using manned or unmanned underwater vehicles or swimmers which have no dynamic stability and travel along random sinusoids. Second, it is not possible to join together as a mosaic, especially for stereophotogrammetric restitution, the strongly pincushion-distorted pictures delivered by the old non-corrected lens with flat glass portholes.