The obvious way to test for deformation of the edifice was to make repeated measurements to targets on snow-free parts of the volcano. Quickly a standard theodolite used by most surveyors was obtained and angles measured to prominent natural features on the volcano. The data suggested outward movement of the north flank, but aiming on natural features was difficult and subject to considerable error. Within a day or two an electronic distance meter (EDM) arrived, and reflectors were placed on the mountain. The EDM is a sophisticated instrument, but the targets were nothing more than clear plastic highway reflectors screwed to boards that were lashed to steel fence posts hammered into the ground. A combination of the theodolite and EDM measurements to these targets soon indicated that the north flank was moving northward at a steady rate, so that clearly the volcanic edifice was deforming in response to intrusion of magma (Lipman and others, 1981). The theodolite measurements to the wooden targets provided sufficient data by themselves to define the rapid displacements of as much as 2.5 meters/day. The EDM data were useful adjuncts to the theodolite measurements but were not necessary to trace the movements at all but the least sensitive targets.
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