In view of all the above constraints, it was decided to adopt an 
analytical method of analysis, incorporating self calibration on site for 
the underwater cameras. The general flexibility of analytical methods to 
cater for a wide range of imagery is well known, as is their higher 
inherent precision. The ability of analytical methods to identify and 
compensate for systematic errors in the photography, particularly 
systematic errors which may vary with submersible locality (e.g. the 
distortion pattern) makes them a very powerful tool. Further, photograph 
measurement can be performed using simple, relatively portable measuring 
instruments, Such as comparators, which can be equipped with digitisers 
and interfaced to computers to enable results to be computed quickly and 
efficiently. 
Two such self-calibration techniques are: the Direct Linear Transformation 
(DLT) solution described by Abdel Aziz and Karara (1971) and the 'flexible 
method' based on collinearity by Moniwa (1976). At an early stage of the 
project it was decided, initially, to concentrate on the DLT method and to 
incorporate the constraint equations proposed by Bopp and Krauss (1977). 
These two constraint equations reduce the eleven dependent parameters of 
the basic projective transformation to nine independent ones. Full details 
of the mathematics involved can be found in the relevant literature. 
A PROPOSED SYSTEM 
The proposal for an offshore based photogrammetric measurement and analysis 
system suitable for performing routine underwater work is illustrated in 
Figures 5 and 6. Essentially there are two distinct components to the 
system:- the underwater data acquisition and the support ship based 
processing and data analysis. 
The underwater component consists of submersible, cameras and calibration 
frame. A pair of UMEL 70mm cameras, which have been modified to include a 
film flattening device, are electronically synchronised and rigidly mounted 
on a pan and tilt unit located at one side of the front of the submersible. 
The unit also carries flash heads, a back up (unmodified) 70mm camera and a 
high quality underwater video camera which can perform video survey and 
also act as an underwater viewfinder for the still cameras. Initially it 
is suggested that the pair of 70mm cameras are mounted 0.4m apart with a 
convergence of 15° and a distance setting of 1.6m which produces a usable 
depth of field between 1.2m and 3m. This allows the cameras to be operated 
in the manner customary to submersible operation. The pilot/observer 
controls the pan and tilt unit and camera firing from within the 
submersible. 
To enable on-site camera calibration, a calibration frame, assembled from 
aluminium bar and resembling a cage of approximately one cubic metre, is 
built into the superstructure of the submersible. This cage is located 
either on the opposite side of the front of the submersible, or along one 
side, in such a manner that it is protected from damage by the submersibles 
barge bar. The frame carries identifiable control points which will be 
pre-coordinated, using a suitable metric camera, prior to underwater work. 
45