flash guns. Initially, the cameras were tripod mounted and the flash guns 
hand held. In railway tunnels, this resulted in having to move a 
considerable amount of equipment by hand for each pair of exposures. 
Although the cameras were still tripod mounted for work in canals, they 
were set up on a barge which was propelled through the tunnel, Plotting 
was carried out on one of our Wild Al0's. Stereometric photogrammetry 
provided a rapid method for measuring tunnel sections, but the resulting 
accuracy did not live up to expectations. It was considered that the main 
reason for the disappointing results was the effect of the accumulation of 
soot on the tunnel walls. The plotting of a profile is in effect similar to 
plotting a contour of a vertical surface from aerial photography, and the 
ability to fuse the images is critical. With dense black walls, as found 
in a tunnel, frequently with a smooth surface, fusing the image was 
difficult to say the least. 
The cost of the operation prohibited the technique being used to survey the 
total number of tunnels in the United Kingdom. Improvements in the speed 
of taking the stereo photographs, resulting in a small cost reduction, were 
obtained by mounting the cameras and flash guns on a trolley for railway 
work. Improvements in accuracy were made by extending camera separation 
to two metres on the base bar, and restricting the camera to section distance 
to a minimum. 
Alternative methods to stereometric photogrammetry have been sought for 
at least the past five years, and B.K.S. joined this search in the Summer 
of 1980. Initial experiments were inspired by the technique used by 
Mr. Tan Waite in the measurement of the cavities constructed for the 
Dinorwic Pumped Storage Scheme. 
Development of the Light Plane Generator 
rr tiet 
  
During the Autumn of 1980, tests were carried out using a prototype light 
plane generator mounted on a Black and Decker Workmate. The light source 
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