Geodetic leveling and the sea level slope along the California Coast

Comparisons between ‘free’ adjusted normal orthometric leveling elevations and mean sea level at tide stations are given by Braaten and McCombs (1963) for the United States coastlines. Their report indicated an apparent systematic difference between the 1963 free adjustment, which had a mean epoch of 1939, and mean sea level along the coasts. Since that time, new leveling surveys, performed primarily by the National Ocean Survey's National Geodetic Survey, have been accomplished between San Francisco and San Pedro tide stations for the epochs 1968–1969, 1968–1971, 1971–1972, 1973–1975, and 1977–1978. The observed elevations at tide stations are compared to mean sea levels of the 1941–1959 epoch. Leveling surveys show very good consistency with a steady trend from negative to positive between 1968 and 1978. This trend indicates that San Pedro is rising with respect to San Francisco or that San Francisco is subsiding with respect to San Pedro at an average rate of about 70 mm/yr. However, an examination of the tidal records does not reveal large changes in the trend of mean sea level with respect to the land at the tide stations. Hicks and Crosby (1972) report that the mean sea level trend with respect to land (1940–1972 series) is −0.3 mm/yr at San Pedro and +1.8 mm/yr at San Francisco. Thus the indicated relative movement rate (70 mm/yr) from leveling is about 30 times greater than the rate indicated by tidal observations. The reasons for the large discrepancy between relative movement rates from repeat levelings and tidal observations are presently unknown.