46 Observations and Measurements of Ocean Waves 



4. Stereophotogrammetric Survey of the Sea Surface 



Kohlschuetter (1909, p. 135) on the "Planet" Expedition was succesful 

 in taking photographic pictures. Since then great improvements have been 

 made, both in the photographic material and in the measurement of the 

 stereo pictures. The fundamental conditions for stereophotogrammetry, which 

 are not always easy to fulfil aboard ships, are : the base-line should have an ap- 

 propriate length and the cameras must be high enough above the sea surface. The 

 great development of (Stereoplanigraph of Zeiss or aerocartograph of Hugers- 

 hoff) instruments to measure the plates have eliminated the necessity of 

 both photographic plates being in the same plane. Now it is only necessary 

 that in the measuring instrument the plates have the same orientation as 

 at the time of the exposure, and give again a good stereoscopic picture. The 

 Zeiss cameras used by the "Meteor" Expedition 1925—7 had a device which 

 consisted in that below to the wave cameras there were two other cameras 

 solidly attached at an angle of 90°. These auxiliary cameras photographed 

 each other (Fig. 28). For each picture they obtained four plates and on each 



main camera 



~~y* auxiliary camera *t~l 



Fig. 28. Position of the main and auxiliary cameras for stereophotogrammetric wave 



pictures. 



plate there was a horizon. There are sufficient points to determine the exact 

 position of the main plates. By evaluating the plates we obtain a topography 

 of the sea surface and we can measure this map in any desired direction 

 (Schumacher, 1939, p. 8; Weinblum and Block, 1935). The size of the 

 field depends on, and increases with, the length of the base line and the height 

 of the camera's above the sea surface. We obtain a plot in the shape of 

 a triangle with the point in the direction in which the picture was taken. 

 This field will be characterized by the height of this triangle (the depth of 

 the field) and by the width of the boundary line in the back. Table 7 gives 

 the length of the basis and the height of the cameras above the sea surface 

 which have been used up till now for stereophotogrammetric pictures. It also 

 gives the error in distance and height which can be expected by the evaluation 

 from a stereoplanigraph. The ideal solution, of course, is to have a base 

 line so long and so high that even in rough seas a picture containing several 

 wave trains can be obtained, which can be measured. One can see, for instance, 

 that a base-line of 6 m as used by "Meteor" Expedition was too small and 

 that, in such a case, it is a matter of luck to have in the field of observation 

 a full wave length from trough to trough or from crest to crest when the 

 sea is rough. The conditions on the "Deutschland" were already much better 

 and made it possible to secure several wave lengths on one picture. The 



