From 1980 to 1989, scientists of the U.S.Geological Survey's Cascades Volcano Observatory established trilateration and distance-measuring networks on Mount Baker, Mount Rainier, and Mount St. Helens in Washington; Mount Hood, South Sister, Newberry, and Crater Lake in Oregon; Medicine Lake, Mount Shasta, Lassen Peak, and Mammoth Lakes in California; and Augustine Island in Alaska. The networks were installed to provide baseline information on potentially active and dangerous volcanoes. The experience gained in monitoring Mount St. Helens has helped in designing guidelines for establishing these networks. One improved method of measuring air temperatures (for atmospheric corrections to the distance) at each bench mark has led to considerable savings in helicopter time by not requiring continuous temperature measurements along the path of the infrared signal. Bench-mark stability is critical and must be carefully evaluated both during installation and later if movement is measured. The equipment and methods currently in use are sufficient to meet the precision (+/- 2-3 ppm about any given distance) that is required for baseline data.
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