e it unrolled,
tments were
1 and ioniza-
become non-
ng from an
velopment of
as, In 1937
~quired, with
photographs
actory. For
nd time were
long junction
Canadian Air
— an arrange-
es for experi-
rews received
photographs
1g in Canada,
’ommittee on
IacNaughton
ibited a keen
way. On the
yyal Canadian
d well-staffed
and the con-
he application
by the close
jonsoring and
t of different
such subjects
photographic
-dinate purely
k in close co-
t the develop-
rogress in the
»f photogram-
search Labora-
lied problems
—G
——
(9)
HORIZONTAL CONTROL
Control for Canadian air photography has always been a difficult problem.
In settled districts traverses and levelling have been possible along roads or
railways, but in enormous areas this is not possible. Much photography in the
north can only be tied in with the planimetric position of points readily identified
in the photographs and established by astronomic observation. This method
has limitations arising from plumb line deviation and, in addition, it is expensive
and often time-consuming. A minor problem in the north is the visibility of
small stars against the relatively bright summer sky in the case of telescopes of
medium aperture.
In the interest of economy, attention has always been given to wide-angle
photography and the "bridging" of as many photographs as possible between
control points. Graphical radial triangulation was first used. Later in 1934,
Maj. General E. L. M. Burns and R. H. Field, working with the Surveys Research
Committee, designed the Radial-stereo plotter for carrying out the same opera-
tions instrumentally. Still later, in 1937, a second instrument, the Stereograph,
was designed with the same end in view. While these machines were an improve-
ment on hand methods, they failed to yield the required accuracy of interpolation,
due to tilt and other effects. It had been foreseen that three dimensional
observations were essential in carrying forward accurate minor control, and in
1933 Imperial Oil Limited, of Canada, placed a sum of money at the disposal
of General MacNaughton for the acquisition of a stereo plotting machine to
aid in the development of Canadian photogrammetry. After deliberation bv
the Surveys Research Committee, it was decided to purchase a plotter designed
by Brig. Hotine of the British War Office, and embodying the ideas of the
South African. photogrammetric pioneer, Fourcade. This machine was com-
pleted in 1939, but, unfortunately, a few months later was totally destroyed by
enemy action at Southampton, England, before it could be shipped to Canada.
In 1947 a sub-committee of the Surveys Research Committee commenced
work on the application of Shoran for extending major control from geodetic
stations into unsurveyed territory. Many experimental difficulties had to be
overcome, but application on a practical scale was initiated in 1949 and con-
tinued in the following years in central Canada and has now reached an axial
length of 2500 miles (4,000 km) extending from the 49th parallel in 5. E. Manitoba
northwesterly to the mouth of the Mackenzie River at the Arctic coast. To
date the trilateration is composed of 149 measured lines of which the average
length is 210 miles (340 km) and the maximum length 329 miles (530 km).
Forty-two stations have been established in the network with an accuracy
much superior to that of astronomic fixation, the only other feasible and economic
method now available in the hinterland. Concurrently, some progress has been
made in using Shoran to fix the position of an aircraft during photography.
VERTICAL CONTROL
Many experiments have been undertaken in Canada for extending vertical
control for mapping from air photographs. In country where a traverse could
be run along roads or railways, P. E. Palmer used aneroid barometers together