Recent investigations show that the refraction corrections in leveling are large but can be adequately modeled. The corrections are proportional to the temperature differences and the square of the sight lengths. Changes in sight length with time have resulted in apparent crustal movements when non‐refraction‐corrected data were used. Applying refraction corrections to leveling data in southern California significantly alters the size and configuration of the southern California uplift. Uplift at Palmdale and Gorman‐Lebec relative to Los Angeles (BM V32) during the 1953–1978 time period is no greater than 10 cm. The bulk of the motion, about 6 cm, occurs as localized motion along the San Gabriel fault during the 1953–1964 time period. This motion along the San Gabriel fault and uplift of 8 cm directly north of the San Fernando fault between 1955 and 1968 may represent precursors to the 1971 San Fernando earthquake.

Large station displacements observed from Imperial Valley global positioning system (GPS) campaigns are attributed to the November 24, 1987, Superstition Hills earthquake sequence. Thirty sites from a 42‐station GPS network established in 1986 have been reoccupied during 1988 and/or 1990. Displacements at three sites within 3 km of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (MS 6.6) within a preexisting GPS network. Best‐fitting uniform slip models of rectangular dislocations in an elastic half‐space indicate 130 ± 8 cm right‐lateral displacement along the northwest‐trending Superstition Hills fault and 30 ± 10 cm left‐lateral displacement along the conjugate northeast‐trending Elmore Ranch fault. The geodetic moments are 9.4 × 1025 dyn cm and 2.3 × 1025 dyn cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest that postseismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using singular value decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of nonseismic secular displacement is observed across the Imperial Valley, which is attributed to interseismic plate‐boundary deformation.

A field test designed to measure atmospheric refraction error in historical and modern leveling was conducted in May–June 1981 on a 50‐km‐long grade from Saugus to Palmdale, California. During 1955–1971, the length of sights made between the level instrument and rods systematically decreased from 60 m to 26 m. The difference in height near Palmdale measured by single‐run long‐sight (42‐m) and short‐sight (22‐m) leveling during the test was 6 times larger than expected random error. Correction for refraction by using either the observed or modeled vertical temperature gradient in Kükkamaki's balanced sight equation reduced the height difference to the level of random error uncertainty. The observed temperature gradient obeyed a power law relation, T = a + bzc, where z is the height and a, b, and c are constants that depended on atmospheric conditions and the ground surface beneath the line of sight. The refraction‐corrected leveling satisfies the specifications and meets all standards of first‐order control surveys. The six historical surveys of the Saugus‐Palmdale grade were corrected for refraction error using the results of the experiment and for rod scale errors and nontectonic subsidence considered in previous investigations. The corrected uplift near Palmdale reached 56 ± 16 mm with respect to Saugus during the period 1955–1965. This amount of uplift is about one third that obtained before removal of refraction error. The corrected displacement profiles also reveal previously unrecognized deformation in the epicentral region of the 1971 San Fernando ML = 6.4 earthquake during the decade before the main shock.

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