Electronic methods allow the establishment of such a net in a very short time. In com 
parison with the usual first order geodetic triangulation the distances between ground 
control points established electronically are much longer and in average amount to 
350 km. There are several electronic systems but in order to simplify our report, only 
one of them called Shoran (or its improved version Hiran) will be outlined in a few 
words. 
In this method a unit installed in an aircraft sends electromagnetic pulses, which 
are received and transmitted back by the ground stations. The time which elapses from 
the instant of sending of signals until its receipt is measured electronically and conver 
ted to a distance. Aircraft and two ground stations form instantaneously a triangle. 
When the aircraft crosses the line joining two ground stations the triangle lies in a 
vertical plane. The sum of distances from the aircraft to the two stations is then a 
minimum and its value reduced to the geodetic reference surface represents the distance 
between the ground stations (Fig. 1). 
In order to increase the accuracy of distance measurements, the cross-flights of the 
same line are repeated several times and on different days. The Shoran operation carried 
out systematically, e.g. across Canada proves that the relative error of distance determi 
nation is of the order of 1 : 60000 for an average distance of about 300 km. This relati 
vely great distance between ground stations must be maintained because the error is 
constant. In consequence, the relative error is greater for small distances. 
Shoran’s network is built up from distances (sides of the triangles) and not from 
the angular measurements as is the case in classical triangulation. Astronomic azimuth 
determinations provide the proper orientation of the Shoran net. It is obvious that the 
accuracy of Shoran points is much higher than the accuracy of astronomically determi 
ned points. 
From this point on, there are two possible ways to complete proper photogrammetric 
mapping operations. The first is based on further use of Shoran (or similar techniques) 
to determine the position of the aircraft at the instant of photographic exposure. 
If the coordinates of two ground stations are known, the simultaneous distance mea 
surements between those stations and the aircraft will determine the position of the 
aircraft at any given moment. In practical operations in order to obtain necessary pho 
tographic coverage with required accuracy supplementary ground stations must be first 
established by the previously described technique. Then, the usual photographic flights 
are carried out with simultaneous determination of aircraft position at the moment of 
each photographic exposure. The required density of ground stations is one station for 
about 30000 km 2 . This is still a relatively sparse net. 
Range of flights controlled by two stations is limited by following conditions: the 
intersecting angle must be larger than 60° and smaller than 120°, and the distance of 
the aircraft from the stations should be between 65 and 280 km. 
For mapping purposes the horizontal position of the nadir point of each aerial pho