The impact of refraction correction on leveling interpretations in Southern California

Abstract

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.

An empirical assessment of refraction error in leveling as a function of survey order and environment

“…The theoretical basis for an expected accumulation of the URE, and thus the underpinnings for the modeled corrections, has been described in detail by a number of investigators (see, for example, Whalen, 1981;Strange, 1981;Shaw and Smietana, 1983 ). Accordingly, we consider here only the essentials of both the problem and those procedures that have been used in the past in attempting to minimize its occurrence and estimate its magnitude.…”

“…Accepting this premise, the upslope refraction error (.Llu) should be greater than the downslope error (ill) during daylight hours, thereby creating a likelihood for the systematic accumulation of refraction error. The magnitude of the URE (the negative or modeled refraction correction) commonly is approximated as: l:lt' = t 0 _ 5 -t 2 _ 5 (the difference in temperature in degrees Celsius, at 0.5 m and 2.5 m above the ground); and L = sight length (modified from Strange, 1981Strange, , p. 2810Strange, , 2812.…”

“…The cumulative impact of atmospheric refraction on leveling observations, whether systematic or not, traditionally has been dealt with through the imposition of procedural constraints intended to minimize any resultant error to no more than trivial values. Much more recently, attempts have been made to model the URE and thereby provide a means for correcting the field observations (see, for example, Whalen, 1981;Strange, 1981;Holdahl, 1982).…”

Geodetic Monitoring of Tectonic Deformation: Toward a Strategy

“…The implied tilts were on the order of several microradians, well above the claimed accuracy of precise leveling. HeTiever, Jackson et al (1980) and Strange (1981) have suggested that the apparent uplift may be the result of height-dependent systematic errors. Stein (1981) and Mark et al (1981) have defended the tectonic interpretation.…”

“…Concern about the reliability of the usually quoted accuracy for leveling has become apparent recently, particularly for regions of steep terrain and for older data. it has been noted that systematic errors due to inadequate rod calibration and failure to correct the historic data for vertical refraction may have produced anomalous crustal deformation information (Strange-, 1981).…”