34 
A field calibration for the Stereo Bar, 
based on well defined and accurately co- 
ordinated control points, established the 
relative orientation between the two cam- 
eras. It was found that differential angles 
AG, Aw and Ax (ie. swing, tilt and rota- 
tion about camera axis) between the two 
cameras axes were within 6 minutes of arc. 
These constant values can be introduced 
into the Autograph A7 by corresponding 
rotations of its two projectors so that the 
essential geometrical relationship between 
the stereo bar and the plotting machine is 
Fig. 6: Short base stereo bar rep- maintained. The plotting accuracy decreases 
resenting a tilt of +90 degrees with increasing distance from the bar. How- 
and showing arrangement of hori- ever, for distances up to 50 feet errors in 
zontal and vertical circle, of bubble position and height were found to be within 
and counter weights. + inch. 
Since the tilt and swing for each pair of photographs can be set to the nearest 
degree only, one control point is necessary within the coverage of each stereo 
model to allow accurate determination of the absolute orientation in the Autograph 
A7. This requirement, however, can be cut down to a few existing co-ordinated 
points as it is possible to transfer control from one model to the next by selecting 
a pass point in their common overlap. 
Future uses may be found for the stereo bar in the field of short range photo- 
grammetry, e.g. hydraulic model studies, architectural studies, etc. 
C. Aerial Photogrammetry 
The terrain in the Snowy Mountains Area presents a challenge to aeria! mapping 
at a large scale. 
Between 1,000 and 5,000 feet elevation where the main structures of the Snowy 
Mountains project will be located, deep river gorges and slopes of 45 degrees are 
predominant. In addition, dense timber coverage stretches over most of the terrain 
in a few areas obscuring the ground totally for several square miles. The access, 
although greatly improved during recent years, is still difficult. Second order 
triangulation now covers the major projects of the scheme, but the break down for 
air photo control into valleys of main rivers, is still very costly and time consuming. 
Naturally, in such virgin bush country the identification of ground control points 
presents a problem. The majority of requests for mapping has been large scale 
pláns at scales of 1: 2,400 and 1 : 3,960. 
1. Medium Scale (1 : 15,840) 
A small amount of aerial triangulation was carried out to supply control for 
the Contractor’s Multiplex plotter in certain areas where lack of access would have 
resulted in high costs of ground control surveys. 
2. Large Scale (1 : 2,400 and 1 : 3,960) 
(a) Aerial Photography 
Prior to 1949 some low level aerial photography had been undertaken for 
preliminary investigations of selected areas within the Snowy Mountains. Photo- 
graphs were taken with a 6" Williamson Eagle IX camera from flight altitudes 
between 5,000 and 14,000 feet above ground. During the early years of operation 
of the Authority's own Stereo Autograph these photographs provided the basic 
material from which large scale plans were compiled. 
In October, 1955, the first low level aerial photography for the mapping of 
selected areas was requested by the Authority. Within three summer seasons 12 
photographic missions were completed covering a total of approximately 1,500 
square miles. Some of the areas were photographed with a Williamson Eagle IX 
and others with a Wild RCS aerial camera, but for the heavily timbered country a 
normal angle lens which would have been desirable was not available. 
Early in 1958 the Authority bought a de Havilland Beaver aircraft (Fig. 7) 
to serve as a transport and personnel carrying plane between the Regional Construc- 
tion Centres. With a jettison hatch in the floor at the rear of the cabin offering a 
possibility for aerial photography, an English F24 camera with “Ross 5 inch wide 
angle” lens was acquired. This camera, featuring a focal plane shutter which makes