Scientific Investigations Report 2007–5251
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5251
Version 2.0, June 2013
GPS and InSAR measurements can be compared at monuments MAGF, MANI, and OSDO, and at the CGPS station COTD in Palm Desert (figs. 3, 5, 6, 7, 8). InSAR data are uninterpretable at the other monuments in the land-subsidence monitoring network and at CGPS station TMAP (near S753) because they are in and near agriculturally active areas (which degrades the InSAR data). Because SAR data and GPS data for MAGF, MANI, and OSDO were not collected simultaneously, magnitudes of land subsidence measured by the two methods could not be compared; instead, rates of subsidence were compared at these monuments. Rates of land subsidence that were derived from GPS data measured at GPS monuments were calculated by differencing the ellipsoid heights computed from the GPS surveys in 2000 and 2005 and dividing by 1,813 days—the number of days between August 30, 2000 and August 17, 2005. Rates of land subsidence derived from InSAR data for the locations of GPS monuments were computed by determining the magnitude of land subsidence for each interferogram near each monument, then dividing that magnitude by the number of days that the interferogram spans; 22 subsidence rates (between February 26, 2003 and September 25, 2005) were calculated from the 22 interferograms analyzed. At COTD, absolute magnitudes of subsidence derived from GPS data and InSAR data were compared because continuously collected GPS data permits computation of a position for any day of interest.
At MAGF, 138 mm (0.45 ft) of subsidence was calculated from the GPS data, resulting in a calculated subsidence rate of 0.08 mm/d (2.6 x 10-4 ft/d). The subsidence rates calculated from InSAR data ranged from 0.06 mm/d to 0.14 mm/d (2.0 x 10-4 ft/d to 4.6 x 10-4 ft/d); the average calculated subsidence rate was 0.08 mm/d (2.6 x 10-4 ft/d) and the standard deviation was 0.03 mm/d (9.8 x 10-5 ft/d). At MANI, 46 mm (0.15 ft) of subsidence was calculated from the GPS data, resulting in a calculated subsidence rate of 0.03 mm/d (9.8 x 10-5 ft/d); the calculated subsidence rates derived from InSAR data ranged from 0 mm/d (0 ft/d) to 0.07 mm/d (2.3 x 10-4 ft/d); the average and standard deviation were 0.02 mm/d (6.6 x 10-5 ft/d). At OSDO, 192 mm (0.63 ft) of subsidence was calculated from the GPS data, resulting in a calculated subsidence rate of 0.11 mm/d (3.6 x 10-4 ft/d); the calculated subsidence rates derived from InSAR data ranged from 0.05 mm/d to 0.21 mm/d (1.6 x 10-4 ft/d to 6.9 x 10-4 ft/d); the average was 0.11 mm/d (3.6 x 10-4 ft/d) and the standard deviation was 0.05 mm/d (1.6 x 10-4 ft/d).
The coordinates for CGPS station COTD were computed by SOPAC for the dates May 7, 2003, and September 25, 2005, and were compared to the InSAR time series that spans those same dates. GPS observations indicated 24 mm (0.08 ft) of subsidence, while InSAR measurements indicated 25 mm (0.08 ft).
The calculated subsidence rates for monuments MAGF, MANI, OSDO, and the subsidence magnitude at COTD calculated using both GPS data and InSAR data compare favorably, thereby improving confidence in results derived from both methods. The favorable comparison of data for these monuments lends credibility to the GPS data collected at the other monuments in the network where InSAR data is unusable because of incoherence caused by agricultural land use.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5251
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