(687) 
from oblique photography only, one of several standard systems of oblique 
measurement are employed. 
The problem of detection of underwater depths from aerial photography 
is quite different from other metrical problems encountered in amphibious 
photographic interpretation and is worth at least a brief separate treatment: 
II. Application of Interpretation Techniques in depth determination. 
The techniques of underwater depth determination have been described by 
various writers. Only the principal methods of proven operations value will be 
DEPTH DETERMINATION 
WAVE WATERLINE PARALLAX 
METHOD METHOD METHOD 
(High Speed) 
^ 
      
  
; es 
?: Succesawe | 
1i Sorties 
7 
     
Stereoscopic 
Measurement 
  
  
  
PERFORMANCE PERFORMANCE PERFORMANCE 
ACCURACY VARIES CONDITIONS = ACCURACY GOOD - METHOD ACCURACY EXCELLENT == GIVES SPOT 
GIVES AVERAGE DEPTHS ONLY RELIABLE EVEN WITH POOR PHOTO- DEPTHS 
IMITAT GRAPHY GIVES SPOT DEPTH 8 
LIMITATIONS CONTOURS LIMITATIONS 
REQUIRES WELL DEFINED WAVES REQUIRES TRANSPARENT WATER 
REQUIRES ACCURATE TIMING LIMITATIONS REQUIRES BOTTOM DETAIL 
LIMITED TO HIGH TIDE RANGE AREAS REQUIRES SUN ANGLE 35%55° 
| REQUIRES A NUMBER OF SORTIES REQUIRES EXPERT PHOTOGRAPHY 
| GIVES RESULTS ABOVE LW ONLY 
Fig. 7. Diagrammatic summary of 3 methods of water depth determination. 
briefly outlined here. The reader is referred to the more comprehensive treat- 
ments by Lundahl (3), Williams (2), and others for further information. 
Wave Metbods. Of the wave methods, the *wave velocity," and, the 
“wave period” (or “wave length”) methods are most frequently used. Another 
method, called the “wave refraction” method has not been used to any extent 
in practice. 
The wave velocity method is based upon the relationship between the 
velocity of a wave moving over a sloping bottom and the depth of the water 
beneath the wave. Practical application of the method requires the taking of 
accurately timed vertical photographs of a suitable wave pattern approaching 
the beach. 
Timing is usually provided by mounting a clock with a sweep second hand 
in the camera so that the dial is recorded photographically in the corner of 
each exposed frame. The velocity of each wave is determined by (1) measuring 
53a