Submarine Tubes for Levelling

Abstract

THE ocean surface deviates from the geoid, that is, from a level surface, by an amount probably exceeding one metre even if short-period disturbances such as waves and tides are eliminated. It therefore cannot serve as a datum for precise levelling. Coasts separated by water can, however, be joined by hydrostatic levelling, recently described by Nørlund. In 1938 he 'bridged' the Store Belt¹ in Denmark with a water-filled submarine tube of length 18 km. and inside diameter 1 cm ; the greatest depth was 60 m. Observations of atmospheric pressure at each end and of temperature distribution along the tube served to determine the difference in level between the water surfaces inside each end. In 1939 he joined the spirit-level networks of Deumark and Sweden by means of a 4.2 km. submarine tube beneath Oresund².