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Stringent accuracy specifications required that particular care be taken in the provision of geodetic control for the construction of the Superconducting Super Collider. The design and implementation of the surface geodetic control networks are described. The horizontal network was realized by Global Positioning System measurements, and the vertical network by means of precise geodetic leveling. The highly expansive soils in the project area required that the horizontal and vertical surface monuments be designed to be stable for the duration of the construction of the project. The methodology adopted for the densification of horizontal and vertical control from the surface networks to the shaft area, and an innovative technique used for the transfer of control into the underground facilities are described. The underground networks were designed so as to minimize the geometric weakness inherent in tunnel surveys. In the interests of economy, removable tunnel brackets were developed and used for the large number of horizontal underground monuments. Survey results for five tunnel half-sector breakthroughs are reported.

The accuracy of Global Positioning System (GPS) relative positioning depends on the geometric distribution of the observed satellites and on the quality of the observations. Different types of errors affect GPS relative positioning in different ways. Some of the errors may have systematic effects on the measured baselines, which may produce significant scale errors and rotations. These effects, however, may change from one survey epoch to another due to a change in the error characteristics and/or the sky distribution of the observed satellites. Therefore, they cannot be completely canceled out in the computation of deformations. In monitoring and engineering surveys of high precision, these effects must be modeled and eliminated. This paper discussed the effects and their modeling. As an example, a GPS ground-subsidence monitoring network in Venezuelan oil fields was analyzed. Rotation of up to 6 ppm in a vertical plane was obtained between survey campaigns in 1990 and 1991. To significantly minimize these systematic effects, some recommendations are made for the design of monitoring schemes and control networks for large engineering projects.