Geologist John Ewert using a laser surveying instrument to measure distances to reflective prisms or targets installed on the flanks of Redoubt Volcano in the distance. Minute changes in distances to the targets can indicate ground deformation that may be related to subsurface magma movement. View is to the south. Photograph by R. McGimsey, May 4, 1990.
Image courtesy of AVO / U.S. Geological Survey.