pole protruded outwards and upwards at an angle. A fluid 
head for a video camera could be slipped over the top of 
the pole. The UMK was attached to this head by means 
of a specially manufactured plastic mount. Thus the 
camera could be tilted and swivelled for pointing at the 
wall and then the camera body lowered so that the back of 
the camera was uppermost to facilitate changing of plates. 
The design of the mount was such that most of the weight 
of the camera was borne by the operator’s stomach and 
not by his arms thus reducing strain. A simple wire open 
sight to aid pointing was manufactured and slipped over 
the lens cone. None of the UMK’s normal electrical 
system was incorporated in this mount so that the fiducials 
and shutter had to be fired sequentially by hand. 
  
Figure 1 Original design of UMK mounting plate 
One of the largest sources of concern was the choice of 
emulsions. The UMK is not designed for aerial 
photography and therefore the lens aperture does not open 
sufficiently to allow the fast shutter speeds required for 
aerial work. It was essential that a fast emulsion could be 
used to allow for sufficiently fast shutter speeds to obtain 
sharp images. The range of emulsions available on glass 
plates for the UMK is limited. Therefore a compromise 
had to be made between high resolution so that the targets 
would be satisfactorily imaged and a high enough film 
speed so that vibration in the helicopter would not produce 
unacceptable blurring of the image. Initially it was 
decided to use Kodak T-Max 100 plates which could be 
pushed to 400 ISO in subsequent processing. These were 
available in 5 inch by 7 inch (127 mm by 178 mm) 1 mm 
thick plates by special order from Kodak in the United 
States. For the Linhof, Ilford HP5 panchromatic on 5 inch 
(127 mm) NATO standard rolls were purchased. Delivery 
of the plates from Kodak was quite time consuming so that 
very little time was available to test them between delivery 
and the first flight. 
À major problem was in the design of the monitoring 
targets to be used. Obviously the photogrammetric 
requirement was for a high contrast target of sufficient size 
that would guarantee its imaging on the photography from 
the proposed maximum taking distance of 150 metres. 
However, aesthetic requirements imposed by the Welsh 
Office meant that the walls could not be covered with 
many large unsightly targets. A number of designs were 
    
   
   
   
    
   
  
  
  
  
  
  
    
    
    
     
   
  
    
  
    
    
  
    
   
    
    
   
   
   
    
         
produced and tested using the 300 mm UMK hand held at 
representative taking distances. The final choice of target 
was a white diagonal cross 10 mm thick on a 60 mm square 
black background. 
The targets were to be positioned on a number of vertical 
rows of panels on each wall. Five targets on each panel, 
one in each corner and one in the centre, would enable any 
rotations of individual panels to be detected. Additional 
targets were to be positioned on the bare rock faces above 
and beside each portal close to the heads of rock anchors 
for the detection of any movement in the original rock. 
The positioning and mounting of the targets were the 
responsibilty of TRRL and the Consulting Engineers, R 
Travers Morgan and Partners. 
  
Figure 2 300 mm UMK on original mount 
Some network design was undertaken. After a series of 
simulations it was decided that three runs of photography 
were required. Two runs would be taken close in to the 
wall, with one run being level with the base of the wall at 
a distance of 90 metres from the wall and the other run 
being taken 100 metres higher at a distance of 70 metres. 
A third run would be taken from a mid level position at a 
range of 150 metres to provide overall coverage of the 
walls and to provide a good connection between rows of 
targets. Using a priori standard deviations for the plate 
observations of +/- 3 microns these simulations indicated 
that a precision of coordination of the targets of the order 
of +/- 1 mm in all three coordinates would be achievable. 
3. ACQUSITION OF PHOTOGRAPY 
Shortly before the first photographic sortie the local Police 
informed the contractor that they would require the road 
to be closed when the close-in photography was being 
taken as they were worried that traffic accidents would 
occur with motorists’ attention being on the helicopter and 
not on the road. Additionally they stipulated that the road 
could only be closed for a maximum period of ten minutes 
and they reserved the right, if traffic backed up 
excessively in that time, to order the helicopter off and 
reopen the road before the ten minute slot was up. Two 
slots were allocated at 10.00 hrs and 13.00 hrs. Therefore