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Baseline lengths and geocentric radii have been determined from GPS data without the use of fiducial sites. Data from the first GPS experiment for the IERS and Geodynamics (GIG '91) have been analyzed with a no‐fiducial strategy. A baseline length daily repeatability of 2 mm + 4 parts per billion was obtained for baselines in the northern hemisphere. Comparison of baseline lengths from GPS and the global VLBI solution GLB659 (Caprette et al. 1990) show rms agreement of 2.1 parts per billion. The geocentric radius mean daily repeatability for all sites was 15 cm. Comparison of geocentric radii from GPS and SV5 (Murray et al. 1990) show rms agreement of 3.8 cm. Given n globally distributed stations, the n(n ‐ 1)/2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well‐defined center of mass. Geodetic information can be obtained by examining the structure of the polyhedron and its change with time.

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Coastal zones have changed profoundly during the 20th century with growing populations and economies (Figure 1.2 ). Increasing urbanization was a major driver of this change. Today, many of the world ' s megacities are situated at the coast. At the same time, sea level has been rising and is projected to continue to rise further. However, coastal developments have generally occurred with little regard to the consequences of rising sea levels, even in developed regions such as Europe (Tol et al. 2008 ). An improved understanding of sea -level rise and variability is required to reduce the uncertainties associated with projections for sea -level rise, and hence contribute to more effective coastal planning and management.The preceding chapters have provided an overview of our understanding of sea -level change. These chapters benefi tted from the discussion of position papers at a workshop on sea -level rise and variability held under the auspices of the World Climate Research Programme (WCRP) at the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientifi c and Cultural Organization (UNESCO) in Paris in 2006. The workshop was attended by 163 scientists from 29 countries representing a wide range of expertise and supported by 34 organizations. The workshop prompted and underpinned new research initiatives. This chapter provides a synthesis of the fi ndings and recommendations from this community discussion, including a summary