Geodetic ground-tilt studies have been used to monitor volcanic activity since the 1960's. Development of more precise theodolites and high-quality, compact, lightweight electronic distance meters (EDM's) in the past 10 years has brought about a dramatic increase in the use of trigonometric leveling for high-order vertical control studies. A system for measuring ground tilt trigonometrically, employing a theodolite, EDM, and target/prism sets has been developed for use in volcanologic studies. This system enables the measurement of single-setup bench-mark arrays as much as 4 times larger than is possible with a level instrument at a single instrument setup site. The system is more compact and thus easier to transport in remote or rugged terrain than is the equipment for classical leveling. Data reduction is slightly more involved but is still simple if a programmable calculator or portable computer is used. A comparison of the two methods on the same triangle shows them to be approximately equivalent in their ability to measure ground deformation.
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