ABSTRACT
These 1974 computations combined data from the British and French national triangulations with cross-channel observations made in 1951 and distance measurements from a European traverse of 1971. In 1986 the OS and IGN were commissioned by Transmanche-Link (TML) to update the Channel Tunnel Grid, re-evaluate the relationship between the national reference levels, and provide improved horizontal and vertical control on both sides. The basic geometry of the original CTG was retained, but the 197 4 network by which it was manifested was considered less than adequate. A larger network was computed, incorporating existing secondary observations and other more recent data, including astronomic azimuths. To provide usable control points in the works areas some new surface stations were established to form a local network covering the Folkes tone and Shakespeare Cliff sites (Figure 2), and the relevant observations were included with the previous data in a simultaneous least-squares readjustment. This computation provided revised plane rectangular coordinates in the CTG system, but to avoid confusion these were altered by adopting a shifted origin and this new network, now described as defining Channel Tunnel Grid 1986 (CTG86), was used for the initial design and construction.
