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(688) 
the distance a given wave crest has advanced between successive exposures, and 
(2) dividing this distance by the time interval as recorded on the clock. The 
conversion of velocities to depths is usually made by means of graphs worked 
out for the purpose and available in various reference publications. 
The wave period method was developed to make use of photography of 
beach areas taken by ordinary reconnaissance cameras without internal clocks. 
Like the previous method, it is based on the equation expressing velocity-depth 
relationshop, rewritten in terms of wave period (as equal to wave length 
divided by velocity). 
In using the wave period method the average wave length in a known 
offshore deep water area is first carefully measured from the photography. 
Using the determined wave length and known depth, the wave period is found 
in a graph. Since this wave period may be considered to be constant for the 
area, the same graph may be used to translate the measured lengths of waves 
on other parts of the same photography into water depths. 
The so-called wave refraction method is based on the fact that wave crest 
approaching a straight coast diagonally is refracted as it reaches shallow water 
until it is parallel to the shore. It can be shown mathematically that the change 
in angle of this crest bears a definite relationship to water depth. 
Waterline Method. In areas of high tidal range, bottom contours in the 
tidal zone may be accurately plotted by taking carefully timed sorties at 
varying tide levels. From this series of photographs, a succession of contours 
can be transferred to a mosaic or master map. Obviously, this method is not 
applicable for areas below the low water line or in areas of negligible tidal range. 
It is, however, as reliable and as accurate as the tide-height prediction on which 
it is based. It was widely used in World War II. 
Parallax Method. Depth determination by parallax is simply an extension 
of the parallax techniques used by photogrammetrists in topographic mapping. 
However, because of the need for one to two feet accuracy in depth determina- 
tion, and the greater difficulty in placing a *floating dot" accurately on the 
bottom and on the water surface, it is necessary to use low-altitude, extremely 
large scale (1/500, plus or minus) photography. During the war the Sonne 
continuous strip camera was widely used for this purpose. This method requires 
fairly smooth and transparent water, good bottom detail, and low altitude 
flying, but provides, under optimum conditions, depths accurate to one or 
two feet accuracy at any point in the area covered. It requires special equipment 
in the form of a continuous strip viewer with parallax-measuring attachment. 
III. The Interdependency of Visual and Metrical Analyses. 
It is emphasized that, while we have discussed visual and metrical analyses 
separately, they are closely intertwined in practice. Let us look at a simple 
example. An interpreter sees a white line running from the beach across the 
terrain. He tentatively identifies it as a road. His next step is to measure its 
width as approximately 25 feet which confirms his identification. Then on larger 
scale coverage or under higher magnification, he examines the image carefully 
along its length, noting that a change in tone between the road proper and the 
shoulders can be distinguished in spots. He carefully measures the width between 
shoulders at places along the road where this distinction can be made and, based 
  
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