measurements are made on the left and right hand photograph to common 
identifiable points such as control targets, but for the purpose of taking 
measurements to a space body in which the possibility of mis-identification 
of the common left and right image is likely then it would be sensible for 
observations to be made in a stereoscopic mode. 
The problem of common image identification was overcome by designing a 
double electronic cursor mounted on a bar which, when activated, introduced 
a slight delay in the 'reading' of the left and right hand digitised X-Y 
co-ordinates. A small electric motor allowed for the movement of the right 
hand cursor down the bar to cater for the change of x parallax of common 
images. The normal etched cross was replaced by small red dots. The 
apparatus was, in fact, an electronic parallax bar but which had the added 
advantage of being able to record y parallax in addition to x parallax (Fig 
5). To allow for stereoscopic inspection of the pair of photographs an old 
fashioned mirror stereoscope, mounted on its XY rail system, was modified 
to fit over the digitiser tablet. Visual y parallax may be removed by rot- 
ating the viewing optics and large absolute X differences, which are normal 
in very short range covergent photography, is stereoscopically accommodated 
by tilting one or both of the mirrors. Possible large scale variations 
between the left and right hand common images cannot, as yet, be eliminated 
but this problem could be overcome by introducing independent left and right 
hand zoom lens systems into the optics. The combination of the dual cursor 
"parallax bar" and the mirror stereoscope has, in effect, changed the 
digitiser tablet from a mono-comparator into a simple but effective stereo- 
comparator system. The system is pictured in Fig 7. 
CONCLUSIONS 
The results of these tests have shown that the underwater NIKONOS III camera 
equipped with a W-NIKKOR 35mm lens in air and an UW-NIKKOR 28mm lens in 
water fulfils the specified requirements of precision photogrammetry when 
the original negatives are measured on a precision comparator and when paper 
print enlargements (>8x) are measured on a digitiser tablet of resolution 
O, Imm or better. A single camera attached to a self calibrating beam 
splitter device also has the potential of fulfilling the requirements of 
precision photogrammetry. 
ACKNOWLEDGEMENTS 
The writer is particularly grateful to Mr L G Welham a post-graduate student 
of the University of Cape Town, who provided the data and most of the photo- 
graphy, both surface and underwater, to permit the writing of this paper. 
The writer is also indebted to Mr H F Weehuizen, Senior Lecturer in 
Electrical Engineering at the University of Cape Town, who designed and 
constructed the dual cursor and to Mr D C Kirby and Mrs Alice Akhurst of the 
Department of Surveying of the University who respectively designed and 
constructed the image splitter and digitiser stereoscope and drew the dia- 
grams. 
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