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