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Obviously the distance between the CNGS Target at CERN and the LNGS Opera experiment cannot be measured directly and in fact requires the combination of three independent sets of measurements: two to link underground reference points at each site to corresponding points on the surface; and a third to link the surface points at both sites. Until the Opera results raised questions about the speed at which neutrinos travelled, the main alignment concern for the CNGS beamline had been an orientation problem -to ensure that the beamline arrived to within ~100 m of its target at LNGS. GPS measurements at the two sites, and the use of gyro-theodolite measurements in the tunnel at CERN, ensured that the absolute alignment of the beamline was established to the required accuracy. New determinations of the links between the surface and the tunnel were not considered necessary until interest grew in the distance between the sites, at which point additional measurement campaigns were organised in order to further reduce the uncertainty in the distance. Details of all these campaigns and the distance estimates will be given.
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