There are several types of photogrammetric instruments capable of 

 utilizing aerial photography to plot topographic map manuscripts. All the 

 first-order photogrammetric instruments are precision mechanical-optical 

 stereoscopic devices which re-create the three-dimensional view of the 

 photographed area and permit the plotting of horizontal and vertical infor- 

 mation of the terrain onto a map manuscript. The accuracy of this infor- 

 mation is basically a function of the flying height of tlie photographic air- 

 craft and the type of plotting instrument. 



Application to Project SWOP 



Photogrammetric techniques lend themselves to the solution of many 

 non-mapping problems. Thus, the ocean wave data required for the com- 

 plete fulfillment of Project SWOP are readily obtained by photogramm.etric 

 techniques. Because of the nature of the project, however, several unusual 

 problems were introduced. Ordinarily, over the stable terrain, stereo- 

 scopic photo coverage is obtained by the proper exposure interval of the 

 aerial camera during flight of a single aircraft. Since the sea surface is in 

 constant motion^ two photographic aircraft with synchronized cameras were 

 required to "stabilize" the images inthe stereoscopic photo coverage. Also, 

 some known ground control is a requisite for accurate photogrammetric 

 mapping. No such control exists on the sea surface^ therefore, for SWOP, 

 a vessel towing a raft at a known distance was necessary to establish the 

 "ground control". If all other factors are equal, greater final accuracy 



is obtained with lower flying height, and greater area coverage is obtained 



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