Measuring and Monitoring
Measurements of land-surface elevations, aquifer-system compaction, and water levels are used to improve our understanding of the processes responsible for changes in the elevation of the land's surface. 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. Aquifer-system compaction is measured by using extensometers; these measurements have the added benefit of being depth-specific because extensometers are anchored at specific depths of interest. So, while each extensometer measures some fraction of total subsidence, the measurements can help us better understand the depths at which compaction is occurring. The most precise measurements tend to be made using spirit-leveling surveys and extensometers. The least precise measurements tend to be made by using GPS surveying, with CGPS and InSAR measurements falling somewhere in the middle with regard to preciseness.