| Limitations |
The Department of Water Resources makes no warranties, representations or guarantees, either expressed or implied, as to the accuracy, completeness, correctness, or timeliness of the information in this dataset, nor accepts or assumes any liability arising from use of these data. Neither the Department nor any of the sources of this information shall be responsible for any errors or omissions, or for the use or results obtained from the use of this information. A Groundwater Sustainability Agency is not required to use these data, and their use does not guarantee the adequacy of a Groundwater Sustainability Plan that relies on such data. |
| Purpose |
This statewide InSAR subsidence dataset was acquired as part of DWR’s SGMA technical assistance to provide important SGMA relevant data to GSAs for GSP development and implementation. The dataset is formatted to support the production of maps and graphs that show the extent, cumulative total, and annual rate of land subsidence.
Interferometric Synthetic Aperture Radar (InSAR) is a satellite-based remote sensing technique that measures vertical ground surface displacement changes at high degrees of measurement resolution and spatial detail. TRE processed Sentinel-1A InSAR data over the study area between January 1, 2015 and October 1, 2020 and calibrating them to data from the regional network of Continuous Global Positioning System (CGPS) stations. TRE provided the resulting time series data of vertical displacement values for point locations on a grid with 100 meter spacing, with values representing averages of vertical displacement measurements within the immediate 100 by 100 meter square areas of each point. Gaps in the spatial coverage of the point data are areas with insufficient data quality. The period of record for the point time series data varies by area, starting as early as January 1, 2015 and as late as June 13, 2015. TRE also provided 2 sets of GIS rasters; annual vertical displacement and total vertical displacement relative to the common start date of June 13, 2015, both in monthly time steps. An Inverse Distance Weighted (IDW) method with a maximum search radius of 500 meter was used to interpolate the rasters from the point data.
Towill Inc. (Towill), also under contract with DWR as part of DWR’s SGMA technical assistance, conducted an independent study comparing the InSAR-based vertical displacement point time series data to data from CGPS stations that were not used for calibrating the InSAR data, as well as CGPS stations that were used for calibrating InSAR data in Northern California. The goal of this study was to ground-truth the InSAR results to best available independent data.
The National Standard for Spatial Data Accuracy (NSSDA), developed by the Federal Geographic Data Committee (Document Number FGDC-STD-007.3-1998), offers a well-defined statistic and testing methodology for positional accuracy of geospatial data derived from various surveying methods including satellite remote sensing. The NSSDA is based on comparison of data from the tested dataset to values from an independent source of higher accuracy. For this study, variation in vertical displacement of California’s ground surface over time, as measured from interferometric synthetic aperture radar (InSAR) satellites, was statistically compared to available ground based continuous global positioning systems (CGPS) data.
Tested: 18 mm vertical accuracy at 95% confidence level.
As tested by the processes described, this analysis provides statistical evidence that InSAR data accurately measured vertical displacement in California’s ground surface to within 18 mm for the period January 1, 2015 through October 1, 2020. This statement of accuracy is based on the assumptions that the number, distribution, and characteristics of CGPS check point locations provide a representative sample of the entire study area and of the entire InSAR dataset, and that the CGPS data constitutes an independent source of higher accuracy. This statement of accuracy applies to the state-wide dataset and may vary for regional or localized area subsets.
The Department of Water Resources makes no warranties, representations or guarantees, either expressed or implied, as to the accuracy, completeness, correctness, or timeliness of the information in this dataset, nor accepts or assumes any liability arising from use of these data. Neither the Department nor any of the sources of this information shall be responsible for any errors or omissions, or for the use or results obtained from the use of this information. A Groundwater Sustainability Agency is not required to use these data, and their use does not guarantee the adequacy of a Groundwater Sustainability Plan that relies on such data. |
