(6) 
appears feasible to complete the arch in three more seasons to a junction with 
first-order triangulation in the east on the north shore of the Gulf of St. Lawrence. 
The shoran trilatération is of necessity the first step, and auxiliary stations will 
have to be established in any area concurrent with its use for shoran-controlled 
photography. The lines in general are too long for signal reception unless working 
within 150 miles of each antenna, and in many areas they are not properly placed 
to secure favourable intersection of position. 
Since in geodetic surveying and mapping it is the sea-level distance between 
stations which is required, it is necessary to consider trilatération sufficiently 
strong in a geometrical sense to give not only length to a certain accuracy but at 
the same time to give position to a relatively high degree of accuracy. Triangula 
tion requires the measurement of angles and for a quadrilateral in which all angles 
are measured, four geometrical conditions must be satisfied. In trilatération the 
measurement of the six lines of a quad, imposes only one condition and hence 
there is a falling-off in strength inherent in the method. To offset this, stations 
should be arranged in pentagon pattern so that extra lines of measurable length 
may be introduced from stations in one figure to stations in adjacent figures thus 
giving overlap. We have endeavoured to work to a standard of two lines more per 
station than that required to fix the stations. In practice there are limitations in 
regard to the maximum operating altitude of the line-crossing plane and to the 
distance at which the signals, due to lack of strength, may be received from the 
ground stations. These limits are with our present equipment 20,000 feet in 
altitude and some 165 miles in length. Thus the maximum length of measurement 
is approximately 330 miles. 
Line Crossing anti Length Measurement 
The shoran instrument in the plane flying at uniform ground speed, height 
and direction, in the vicinity of the centre of the line, emits group pulses (20 per 
second) which are alternately sent to the terminal ground stations on frequencies 
of 230 and 250 mgc. and are then delayed a fixed amount and returned on a 
common frequency of 300 mgc. The elapsed time between dispatch and reception 
is available on mileage dials, which together with other dials, are photographed 
at intervals of three seconds. It is evident that the true sum of the distances, 
plane to stations, is greater on each side of the actual crossing and is a minimum 
at the crossing. If time is plotted against sum the plot approximates a parabola 
and a mathematical curve of best fit will determine the minimum sum distance 
corresponding to each line-crossing flight. 
All lines of the project have been measured to a minimum standard of sixteen 
line-crossings, of which two groups of eight each were done on different days to 
ensure a change of atmospheric and other conditions and to reveal unstable 
equipment. 
As an example let us now consider the results obtained for the line Fort 
Simpson—Peace, Tables I-II, which is the greatest length so far measured in our 
operation.