lette, which 
: conversion 
modern aids 
ns extended 
of Victoria 
ho, without 
1 and other 
rit of adven- 
1, familiarity 
requisite for 
as the ‘iron 
> monoplane 
ted on floats 
ge and per- 
te Canadian 
ht as well as 
protection of 
oe roped to 
bin aircraft. 
t for oblique 
ger possible, 
ixed cameras 
ed the design 
ployed in its 
ıld the three 
n the labora- 
' speeding up 
en one of the 
air charts of 
construction 
photography 
rom Mitchel 
photography 
y denuded of 
sible for the 
tri-camera or 
rctic islands. 
rtical photog- 
e it is a large 
re operations 
    
— 
o 
(13) 
were carried out from gravel strip runways, the Dakota took its own camp with 
it. It doubled as a transport and camera aircraft, and helped to ease supply 
problems at critical times. In one operation of an emergent nature, 20,000 
gallons (90,000 litres) of aviation gasoline were consumed in ferrying the 10,000 
gallons (45,000 litres) used subsequently by the same aircraft to carry out its 
prime role of completing a photographic operation. 
During the past decade vertical photography has been used to an increasing 
extent, and improved cameras have become available, notably the Williamson 
O.S.C. and the Fairchild Cartographic, both of which are based on sounder 
designs from a photogrammetric point of view than the cameras previously in 
use. Concurrently, more precision has, been needed in the laboratory calibra- 
tion, and the removal of centering errors in the lenses — which errors undoubtedly 
give rise to some of the troubles in bridging owing to the asymmetric distortion 
introduced. 
All photography is carried out to specifications issued by the Survey Re- 
search Committee, thus ensuring a high standard, particularly in competitive 
contract flying. 
LATER OFFICE PROCEDURE 
As importance shifted from oblique to vertical photography (mapping from 
oblique pictures reached its peak in early 1930) more attention had to be given 
to stereoscopic and other methods for extending minor control. Radial line 
methods were soon applied, the air-base direction being marked in under a 
gridded stereoscope. So great was the number of photographs to be handled 
that it was later found advisable to design a special magnifying stereoscope, 
fitted with a floating mark for this single operation (Ref. 16). At first minor 
control was carried along by tracing angular directions, but later slotted templates 
were introduced and have been increasingly employed. To cut the templates 
a special punch was introduced into the Army Map Service, Department of 
National Defence (Ref. 17). 
Concurrently, the Multiplex (first tried in 1936) was used more and more 
for bridging, following the experiences of Pendleton and his associates of the 
Tennessee Valley Authority, U.S.A. 
Early Multiplex work in Canada was confined mainly to compilation in 
areas where it was possible to establish control in every overlap. Experience 
in wartime military mapping had indicated that it was quite feasible to run 
extensions of ten or twelve overlaps, and early in 1946 several trial extensions 
were made of areas which had previously been controlled by surveys on the 
ground. The results were very encouraging. In models for low relief the 
average error was about 6 ft. (2 m) (plotting scale 1:7500) and in high relief 
areas about 20 ft. (6 m). Empiric formulae were used to compensate for the 
residual errors arising from lateral and longitudinal tilt. 
Long extensions have only been used in isolated instances for 1/50000 
mapping because ground control has been available. However, present com- 
pilation methods make an indirect use of at least two lessons gained in extension