Although steep terrain, difficult access, hazardous conditions, and limited staff often discourage volcanologists from making geodetic leveling surveys at restless volcanoes, leveling has several advantages, including (1) greater precision for vertical displacements than virtually any other method over horizontal distances up to at least 100 km; (2) applications that range from single-setup leveling or short traverses designed to measure local ground tilt to long traverses that measure vertical displacements over broad areas; (3) sufficient redundancy based on field procedures and the large number of bench marks along a typical traverse to virtually eliminate measurement error and bench-mark instability as sources of uncertainty; (4) well-established procedures, relatively inexpensive equipment, and straightforward data processing; and (5) the utility of vertical displacement data as input for numerical models of the deformation source. Disadvantages include (1) substantial manpower requirements form long traverses; (2) logistical problems imposed by equipment that is difficult to transport to remote volcanoes and impractical to use in dense vegetation; and (3) the relatively slow pace of traverse leveling compared to other volcano monitoring techniques. An experienced first-order leveling crew of four to six people will traverse 3-6 km per day on mean slopes of 10 degrees or less, or 1-2 km per day on slopes of 10-30 degrees. Alternatively, a leveling crew can measure several short traverses or more than a dozen single-setup arrays in a day. The standard deviation of vertical displacements determined from successive first-order class II leveling surveys is 1 mm/km^1/2 x L^1/2, where L is distance along the leveling line (that is, +/-3.2mm/10km or +/-10.0mm/100km). At mafic volcanoes, on the lower flanks of stratovolcanoes, at large calderas, and in terrain between volcanoes along volcanic arcs, long leveling traverses can provide important information on large-scale ground deformation. In virtually any terrain and particularly during volcanic emergencies, short traverses are a useful tool to quickly determine short-term deformation rates.
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