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This volume contains most of the papers which were presented at the Interdisciplinary Symposium No. 4 "Geodetic Features of the Ocean Surface and their Implications" during the XVIII. General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Hamburg, August 1983. The symposium was jointly sponsored by the International Associ ation of Geodesy (lAG) and the International Association for the Physi cal Sciences of the Ocean (IAPSO), and was as such one further step in animpQrtant line of international and interdisciplinary symposia, re lated to the field of Marine Geodesy. Originally the term "Marine Geodesy" was widely understood as "Geodesy in the Marine Environment" and dealt primarily with two as pects: precise position determination at sea and determination of a fine structured marine geoid. However, mainly with the impact of satel lite radar altimeter measurements, a new understanding began to develop: it became evident that the field of Marine Geodesy could not be treated adequately from geodesists alone but that it needed close cooperation with related disciplines such as oceanography and marine geophysics. Symposium No. 4 at Hamburg could demonstrate that this coopera tion has already become a lively reality. The "geodetic features of the ocean surface" don't only reflect oceanographical but also marine geo physical aspects. As such scientists from geodesy, oceanography, marine geology and geophysics came together to present their ideas and to dis cuss questions of mutual interest.
Measuring seafloor motion in shallow coastal water is challenging due to strong and highly variable oceanographic effects. Such measurements are potentially useful for monitoring near-shore coastal subsidence, subsidence due to petroleum withdrawal, strain accumulation/release processes in marine shelves and submerged volcanoes, and certain fresh water applications, such as volcano deformation in caldera-hosted lakes. I participated in a project to develop a seafloor geodetic system for this environment based on an anchored spar buoy topped by high precision GPS. Orientation of the buoy is measured using a digital compass that provides heading, pitch, and roll information. The combined orientation and GPS tracking data are used to recover the three-dimensional position of the seafloor marker (anchor). A test system has been deployed in Tampa Bay, Florida, for over one year, and has weathered several major storms without incident. Even in the presence of strong tidal currents which can deflect the buoy several meters from vertical, daily repeatability in the corrected three- component position estimates is 1–2 cm or better. Except for the rapid motion during the first month after deployment due to settling, other large anchor displacements correspond to extreme weather events, and are likely associated with current-induced scour activity.
Geodesy has undergone technological and theoretical changes of immense proportions since the launching of Sputnik. The accuracy of current satellite geodetic data has approached the centimeter level and will improve by one or two orders of magnitude over the next decade. This bodes well for the application of geodetic data to the solution of problems in solid earth, oceanic and atmospheric sciences. The report Geodesy in the Year 2000 addresses many areas of investigation that will benefit from this improvement in accuracy.