DESCRIPTION:
Geodetic Leveling

Geodetic leveling is a long-established technique used to measure elevation differences between successive bench marks and, by repeated surveys, to measure elevation changes (vertical displacements) as a function of time. Leveling has been used as a geodetic measurement system for more than a century, and has proved its utility at several active volcanoes. However, recent innovations in precise geodesy sometimes overshadow the considerable and enduring strengths of leveling as a volcano monitoring tool, to the extent that leveling is overlooked in situations where it could make a unique contribution. ...

During conventional geodetic leveling, a spirit level or other type of leveling instrument (most modern levels are self-leveling and do not include a spirit level) and a pair of graduated leveling rods (typically 3 m long, graduated every 0.5 or 1.0 cm) are used to measure the elevation difference between permanent bench marks by accumulating the elevation differences between a series of temporary turning pints. The forward turning point (relative to the direction of the traverse) is called the foresight and the backward turning point is called the backsight. Typically, bench marks are spaced 1-3 km apart and turning points are 20-100 m apart (depending on steepness of the terrain). Adjacent turning points bound a setup and adjacent bench marks bound a section; measurement of a series of sections constitutes a survey.

Leveling surveys can be used to serve two different but often complementary purposes. Long traverses (typically 10 km and longer) measure near-absolute vertical displacements with respect to a distant bench mark or group of bench marks that is assumed to be stable. Short traverses (typically 1 km and less) measure relative vertical displacements among a small group of bench marks, usually for the purpose of determining local ground tilt. Single-setup leveling is a special case in which relative vertical displacements within a small array of bench marks are measured from a single instrument setup to determine local ground tilt (particularly useful when deformation rates are high or surveying time is very limited. Each of these approaches has unique advantages and disadvantages that determine which one is most appropriate for a specific application, ... One effective strategy is to measure short segments of a long traverse periodically, then measure the entire traverse when significant changes are indicated. ...

The following two examples were chosen to illustrate the utility of leveling surveys in volcanic terrain as a tool for studying active magmatic and tectonic processes. Individual traverses range in length from 196 m at South Sister volcano to 193 km at Medicine Lake volcano. Measurement of the South Sister network of four short traverses requires one or two crew-days, while the Medicine Lake traverse requires about nine crew-weeks. Both networks were established as part of a CVO effort to obtain baseline geodetic measurements at each of the potentially active volcanoes of the Cascade Range.

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