37
. When compared with a total of 50 horizontal and 123 vertical ground control
points the average mean square error for the adjusted machine coordinates was
derived as follows:
M. = + 0.08 mm. M, = + 0.10 mm. M, = + 0.09 mm.
As an average each horizontal control point was represented by 2.4 identified
images, each vertical control point had 1.4 identified images.
The plotting runs (up to 20 models per run) were then triangulated using
control given out by the 3 tie runs. 22 horizontal and 64 vertical additional ground
control points scattered within the flight block as well as common pass points
observed within the overlap of adjacent runs assisted in the study of correlations in
the adjustment of the whole block.
Plotting at a scale of 1 : 3,960 with 20 ft. contours covered 75 standard mapping
sheets, each sheet covering two miles by one mile.
(c) Plotting
Photogrammetric large scale plotting for a big engineering project seldom allows
an organised mapping programme as it was planned and carried out for the Lake
Eucumbene.
The detail investigations in the outlay of the Snowy Scheme quite frequently
open up new alternatives for investigations at short notice. To accomplish photo-
grammetric mapping under such flexible conditions calls for the solution of often
unique problems by applying unorthodox and novel techniques. In most instances
low level photography had been available. However, in many cases air photo control
does not exist and cannot be established in time. On such Occasions the operator
and surveyor must work together and call on the experience of both to obtain a
practical and satisfactory solution, of which a typical example is given below:
Stage 1:
An appropriate pair of photographs from the high altitude flight (25,000 ft.) is
selected for orientation in the Autograph A7. The model is scaled and levelled on
the corresponding part of the General Map (4” = 1 mile) which previously had
been compiled on a Multiplex from the same photography.
Stage 2:
The absolute orientation is then refined by checking on:
(i) unidentified trig stations,
the study of trig. diagrams (especially river chains) often reveals a hint
for assuming the position of a trig station within a relative small vicinity
on the photograph. Where trig signals had been built before the date of
photography the shape of the clearing is a good indicator for the station
itself.
(ii) phototheodolite surveys,
a damsite within the extent of the model may have been surveyed by
terrestrial photographs from which identifiable feature (boulders in river
bed, etc.) can be transferred into the aerial photograph.
(iil) existing traverses,
the profile of a traverse sometimes permits location of its position with
regard to the stereo model.
Trig stations are generally available and it has been found that without
having any identifications a reasonable orientation can be found by a process of
successive approximations. Spotheights given out during the Multiplex compilation
for peaks and saddles and selected near the edges help to level the model.
Stage 3:
From this model 4 appropriate photographic images are co-ordinated by
reading their machine co-ordinates in the A7. If justified, their geodetic co-ordinates
are determined by a Helmert transformation.
Stage 4:
On these 4 control points the large scale model covering approximately a
fourth of the area of the small scale model is scaled and levelled.
Stage $5:
If the area to be plotted is small, so that new points should be given out for
the orientation of the plotting sheet (200 ft. — 1 inch) the machine co-ordinates
for two grid intersections are found by transformation.
This procedure has many variations governed by each individual case. With
regard to accuracy it is established that the evaluation of a storage capacity for
instance is less effected by lack of field control than by the hazard of dense timber
coverage. However, it is stressed again that the above method is applied only when
proper field control is not justified in a particular case.
