relationships of bottom-dwelling organisms, e.g. coral, and temporal changes 
therein, are of central interest (Done, 1981). The methods available to the 
marine scientist to gather his data are limited. Direct observation and 
measurement are frequently laborious and inaccurate. Site-sampling using 
a square frame, (quadrat), is a time-honoured approach favoured by many scuba 
diving ecologists. Specimens are recorded on water proofed data sheets and a 
few coarse measurements of areal coverage may be made with a fibreglass tape. 
The Great Barrier Reef contains over 2000 individual reefs, many of which 
extend over several square kilometres. The inadequacy of the quadrat is all 
too obvious! Close range photogrammetry can provide a suitable method for 
data capture over areas up to one hectare. A flying height of two metres 
seems to be a workable maximum due to (i) decreasing visibility and image 
definition with larger camera to object distance and, (ii) the tops of most 
reefs seldom have more than two metres of water coverage. 
Unlike the single camera stereophotography taken by the HDWB at several 
unconnected test sites, AIMS has been photographing with a twin camera rig 
(see figure 3) along transects (or strips) across reefs. In some cases, 
several overlapping strips have been photographed and these are amenable to 
photogrammetric strip and block adjustments. The provision of suitable 
control points for these blocks is a major underwater surveying problem and 
one attempt at an underwater trilateration has been described by the author 
(Fryer and Done, 1981b). 
The strict tilt constraints placed on aerial photography cannot be en- 
forced underwater and the use of mechanical-analogue plotters is consequently 
limited. Relief displacement may also provide problems unexperienced in 
conventional aerial work: a three metre chasm photographed from a flying 
height of two metres will have its image position displaced by more than 10 mm 
along a 24 x 36 mm negative. Even experienced observers have difficulty 
viewing stereocopically such photographs under the simple mirror stereoscope. 
A series of accuracy tests have been performed on the stereopairs taken 
with the twin camera rig comparing a mirror stereoscope and parallax bar 
technique with computerised collinearity equations. Several 200 mm length 
steel bars were placed irregularly down a reef surface at camera to object 
distances varying from 1 to 3.5 metres. Parallax bar measurements were made 
to determine the depth of each bar and their measured lengths were scaled 
169