Technology in 1985: USGS Scientist conducting Electronic Distance ...

USGS Scientist conducting Electronic Distance Measurement (EDM) lines, South Sister Volcano. EDM lines were conducted routinely to monitor these Cascades volcanoes for potential eruptive activity. By measuring the distance between two fixed points, these instruments detect minor swelling of the volcanic edifice that often occurs prior to eruptions. The helicopter is measure air temperature along the line the EDM is reading.