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Elevation or elevation-change measurements are fundamental to monitoring land subsidence, and have been measured by using interferometric synthetic aperture radar (InSAR), continuous GPS (CGPS) measurements, campaign global positioning system (GPS) surveying, and spirit-leveling surveying. The most precise measurements tend to be made using spirit-leveling surveys and extensometers. 

Spirit leveling, GPS, and extensometer measurements tend to be spatially sparse because these measurements are taken at only a few locations. InSAR measurements are spatially dense. Measurement frequency is dependent on study objectives, measurement methods, and subsidence rates. For example, if subsidence rates are slow, extended periods between GPS measurements may be necessary to permit a greater signal-to-noise ratio (enough subsidence needs to occur between measurements to exceed the expected measurement error), or a more accurate method such as spirit leveling may be necessary.

Black & white photo of two USGS scientists conducting spirit leveling in the field

USGS survey party spirit leveling near Colusa, Sacramento Valley, circa 1905. Photo by John Ryan, donated courtesy of Thomas E. Ryan.​​​​​​​

Spirit leveling, the oldest method of measuring subsidence and uplift, derives its name from the primary tool utilized in the process — the spirit level. Spirit leveling is a precise way to obtain data for smaller land areas, and is commonly used along road, railroad tracks, aqueducts, and canals.

Spirit leveling was once a common method of determining elevation. Before the advent of the satellite-based Global Positioning System (GPS) in the 1980s, the most common means of conducting land surveys involved either the theodolite or, since the 1950s, the geodimeter (an electronic distance-measuring device, or EDM). When only vertical position is sought, the spirit level has been the instrument of choice. The technique of differential leveling allows the surveyor to carry an elevation from a known reference point to other points by use of a precisely leveled telescope and graduated vertical rods. Despite its simplicity, this method can be very accurate. When surveying to meet the standards set for even the lower orders of accuracy in geodetic leveling, 0.05-foot changes in elevation can be routinely measured over distances of miles. At large scales, leveling and EDM measurement errors increase. When the scale of the survey is small (on the order of 5 miles or less) and the desired spatial density is high, spirit leveling is still commonly used because it is accurate and relatively inexpensive. Large regional networks warrant use of the more efficient Global Positioning System (GPS) surveying for differential surveys.

 

map showing land subsidence along the Delta-Mendota Canal from 2000-2009 as measuring by spirit leveling

This map illustrates the precise nature of the spirit-leveling method of measurement. The leveling data was collected by the California Department of Water Resources along the Central Valley's Delta-Mendota canal. The inset graph displays historical subsidence data calculated from leveling measurements for a portion of the Canal. These data are from the Delta-Mendota Water Authority and the Central California Irrigation District. (Public domain.)