A slope-distance monitoring network on the flanks of Mount St. Helens was established one month before the major eruption of May 18, 1980. By 1990, the network has expanded to include 6 instrument and 13 reflector stations designed in a star-shaped configuration. The instrument and reflector stations are steel towers 3-4 m high cemented in the ground. The towers are designed to allow measurements to be taken during winter months when snow covers ground marks. For atmospheric corrections, measurements of temperature and pressure at the instrument station only (without corresponding measurements at the reflector station) have proved adequate for the precision required. Prior to dome-building episodes, measurements taken from a station as far as 8 km away can be used as a predictive tool. Seasonal variations are possibly observed on some lines. Taken as a whole, the data obtained from measurements to the flanks after May 18, 1980, show no correlation with eruptive activity.
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