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DESCRIPTION, METHODS OF CONTROL AND APPLICATIONS IN 
AERIAL PHOTOGRAPHIC SURVEYING OF A DEVICE ENABLING 
TWO PHOTOGRAPHS TO BE TAKEN SIMULTANEOUSLY FROM 
TWO DIFFERENT AIRCRAFT WITH AN INTERVAL OF LESS THAN 
5 MILLISECONDS 
by 
J. Cruset, Ingénieur Géographe Institut Géographique National, France. 
The apparatus described below were designed and built by the Institut 
Géographique National to make possible simultaneously aerial photographs 
from two aircraft flying on the same course and at the same altitude. 
The controlling aircraft carries the intervallometer, which activates a 
transmitter of the type designed and used by the I.G.N. for its ground parties, 
and a special receiver. The latter works through a relay to operate the camera 
in the controlling aircraft. The slave aircraft carries only a receiver of the same 
type as that in the controlling aircraft and this works the second camera. The 
only delays which may occur are delays in the relays leaving the receivers and 
a delay in setting the shutter in motion; these may differ in the two aircraft. It 
has been possible to measure these delays very accurately with a precision 
chronograph also made in the I.G.N. laboratories. This chronograph was 
mounted in various ways to study the various contributory delays and the total 
delay. It is easy to make certain that the difference in the delays caused by the 
relays in the two aircraft shall not exceed one millisecond. If the cones and 
distributors (in the portion of the apparatus containing the electro-magnet 
which operates the shutter) are properly matched, the difference in the delays 
in working the shutters is also less than 1 millisecond. To take 5 milliseconds 
as the potential overall lack of simultaneity is, therefore, to err on the safe side. 
The applications in view are: 
1) Plotting the sea-swell which, in the most unfavourable conditions, requires 
a simultaneity of about 1/150 sec. in the two shots. 
2) Coastal surveys which, when carried out by traditional methods, is ren 
dered the more inaccurate because the surface of the sea extends on a 
greater portion of the pair; 
3) Survey of small islets, using mean sea level for orientating the verticals; 
4) Survey of large islands or peninsulas, combining accurate aerotriangulation 
methods with simultaneous photography, the starting and terminal extrem 
ities of the strips being over the sea. 
CONTRIBUTION TO THE STUDY OF THE BLURRING DUE TO 
IMAGE MOVEMENT IN AERIAL PHOTOGRAPHY 
by 
J. Cruset, Ingénieur Géographe Institut Géographique National, France. 
The Institut Géographique National has made a study of the “blur” in 
aerial photographs caused by the movement of the aircraft during the opening 
of the shutter. Experiments have been carried out both in the laboratory and 
in flight.