1. INTRODUCTION
Satellite altimetry has provided accurate measurements of sea level with near-global coverage over the last two decades. Such measurements have led to the first definitive estimates of global mean sea level (GMSL) rise and have made studies of regional sea level change feasible. The relatively short record provided by satellite altimetry, however, does little to answer the question of how the current state of the ocean compares to previous ocean states. Tide gauges have been the primary source of sea level measurements over the last century. While providing long records, the spatial resolution of tide gauges is poor, making accurate estimates of GMSL and studies of regional changes in sea level difficult. Most tide gauges are located in the northern hemisphere around the heavily populated areas of North America, Europe, and Japan. Just a few provide continuous records back to the 19th century.