Geodetic leveling and sea slope along the Southern California Coast: An update

Abstract

Since the apparent sea slope along the Pacific Coast between San Diego, California, and Neah Bay, Washington, was originally reported by Bowie in 1929, numerous points of view, both geodetic and oceanographic, have been expressed concerning this possibility.

In 1978 a dedicated geodetic leveling project “Southern California Releveling Program (SCRP)”; was completed in less than 6 months. The analysis of SCRP data indicated large unexplained loop misclosures and showed different sea slopes (when analyzed along two routes) between Avila Beach and San Diego. Based on the knowledge of systematic errors and biases at that time, definitive assertions could not be made to explain the large loop misclosures and varying sea slope between two fixed coastal points at the same epoch.

In 1981 the existence of a magnetic error in some automatic compensator‐type leveling instruments was detected. By 1987 empirically derived magnetic error constants were estimated for the leveling instruments used in the SCRP network. After applying the magnetic correction, the large loop closures decreased to within allowable first‐order leveling tolerances. Profiles of repeat levelings corrected for magnetic error still indicated that the leveling data contained significant systematic errors, which appeared to be temporal in nature. In fact, the 1968–69 leveling data from San Francisco to San Diego disagreed with the 1977–78 leveling data by 50 cm. However, the 1977–1978 leveling epoch was only 9.4 cm lower than the level surface, which is less than three times the formal estimate of standard error for a first‐order leveling height difference over a distance of 1,040 km.

This indicates that the latest epoch of leveling data (1977–78) corrected for magnetic error agrees with the level surface estimated using local tidal data within the estimated accuracy of the leveling data. Additional studies are required to determine why the repeat leveling still indicated significantly different tilts from San Francisco to San Diego.