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Conventional surveying operations in the vicinity of Boston's Central Artery/Third Harbor Tunnel Project are hampered by large buildings, severe traffic congestion, pedestrians, and access difficulties. However, the use of the Global Positioning System (GPS) made it possible to meet tight scheduling requirements and at the same time produced accuracies that would have been extremely difficult to achieve using terrestrial geodetic techniques. The projectwide primary network comprises 33 stations and 100 quasi‐independent baselines, which vary in length from 150 m to 11,000 m. Trimble 4000SX receivers were used for the measurements. Double‐difference baseline solutions were computed using the broadcast ephemeris. The minimally constrained adjustment of the baseline components yielded an “average” relative horizontal positional accuracy of 3.8 mm plus 1.7 ppm at the 95&percent; level of confidence. A corresponding relative vertical accuracy of 6.1 mm plus 2.1 ppm of the station separation was obtained for the ellipsoidal height differences. A simple model of geoidal height differences was produced and used for the estimation of orthometric heights.