33 
and a precise contour plan up to the proposed top water level could be provided. 
On future occasions when the reservoir is empty or low, the opportunity will 
be taken to inspect the storage and should conditions warrant it new photographs 
will be taken to study any erosion or siltation which may occur in the reservoir. 
5. Short Base Stereo Bar 
In early underground work difficulties were experienced in covering central roof 
sections of excavations with the phototheodolite due to the upward tilt of the 
camera having a limited range. In view of future work of similar nature it 
became desirable to overcome this shortcoming by 
(a) designing a mount that would allow ihe setting of the phototheodolite 
camera to any desired tilt between horizontal and +90 degrees, and 
(b) solving the problem of reconstructing excessive tilts in the Autograph 
A7 the design of which did not take such conditions into account. 
It was found that 
(i) a tilt of +30° could be treated as a tilt of —30° when each photograph 
was placed in the opposite projector and the height 
counter reversed, 
(i) a tilt:of +60° could be treated as a high oblique aerial photograph 
with the Z axis representing height whilst X and Y 
components transferred the position to the drawing 
table, 
(ii) a tilt of +90° could be treated as a vertical aerial photograph. 
In each case the setting of the triangulation lever, diapositive lever, and a respec- 
tive rotation of the dove prisms ensure a "natural" stereoscopic observation, 
whilst appropriate settings of the handwheel gears and the connecting X and Y 
sleeves to the drawing table transform the projection of each stereomodel into 
a "standard case." 
Since plotting at the final stage of this 
process appeared to be feasible, efforts 
were concentrated on the problem of 
taking photographs at these excessive 
tilts. 
In conjunction with the SMA Engineering 
Laboratory's workshop a horizontal bar 
was constructed which had a special 
frame rigidly attached to both ends. Each 
frame was individually made to fit one 
of the two phototheodolite cameras. 
  
Fig. 5: Short base stereo-bar in- 
corporating two Wild phototheodo- 
lite cameras. 
When the cameras are fitted into their respective frames their relative 
orientation is parallel to each other, both axes being at right angle to the bar. 
Their objectives lie on the centre line of the bar at a fixed separation of 4.00 feet. 
The horizontal bar, thus representing a baseline of fixed length, is supported 
in the centre by a vertical axis, around which this baseline can be rotated in order 
to swing the two cameras into any desired horizontal direction. 
A rotation of the horizontal bar around its axis will tilt each camera simultane- 
ously in a vertical plane around its objective thus setting both cameras at any desired 
but common angle of tilt. 
Both angular rotations, i.e. horizontal for swing and vertical for tilt can be 
effected through the full range on a vertical and horizontal circle respectively. The 
initial setting of the bar is controlled by two bubbles which are levelled with the 
aid of four footscrews attached to a solidly constructed base plate. 
For underground work the stereo bar can be centred over any co-ordinated 
survey mark, preferably in the centre of the excavation. From this position 
photographs can be taken covering all four walls and the roof by employing various 
swings and tilts. As this is done from one single position and one initial orientation 
the most part of time taken up by the survey is in the exposure of the photographic 
plates.