Global Positioning System surveying to monitor land subsidence in Sacramento Valley, California, USA

A subsidence research program began in 1985 to document the extent and magnitude of land subsidence in Sacramento Valley, California, an area of about 15 600 km2, using Global Positioning System (GPS) surveying. A brief description of GPS surveying is followed by a summary of each of four surveys between 1985 and 1989. In addition to periodic conventional spirit levelling, an examination was made of the changes in GPS-derived ellipsoidal height differences (summary differences) between pairs of adjacent bench marks in central Sacramento Valley from 1986 to 1989. The average rates of land subsidence in the southern Sacramento Valley for the past several decades were determined by comparing GPS-derived orthometric heights with historic published elevations. A maximum average rate of 0.053 m year−1 (0.90 m in 17 years) of subsidence has been measured.