Comparisons of OPUS and PPP Solutions for Subsidence Monitoring in the Greater Houston Area

Abstract

Global positioning system (GPS) techniques have been applied to study land subsidence in the greater Houston area for over two decades (1993–2016). The free Online Positioning User Service (OPUS) utility developed by the National Geodetic Survey (NGS) has been frequently applied by the local surveying and engineering community for faulting and subsidence monitoring. Some OPUS users often wonder about the accuracy of OPUS solutions compared with the results from more sophisticated software packages preferred by the geodetic research institutions. This investigation compared OPUS solutions with precise point positioning (PPP) solutions. The authors’ results indicate that the earth-centered-earth-fixed (ECEF) XYZ coordinates (IGS08) provided by the OPUS and PPP methods differ by only millimeters in the greater Houston area. In this study, the PPP solutions with respect to IGS08 were transformed to a stable local reference frame, the Stable Houston Reference Frame of 2014 (SHRF14). The superficial displacements derived from the PPP solutions retain an accuracy, estimated by the RMS error (RMSE), of 2–3 mm in the horizontal directions and 4–6 mm in the vertical direction. The accuracy of the OPUS solutions is 1–2 mm worse than the accuracy of the PPP solutions. Overall, the PPP solutions provide a more coherent and slightly higher accuracy compared with the OPUS solutions. However, this minor difference in positional accuracy between the OPUS and PPP solutions has little effect on the evaluation of long-term (e.g., >3 years) site velocities. There are significant seasonal signals superimposed into the GPS-derived land subsidence time series in the greater Houston area. As a result, the estimated linear trend is often sensitive to the length of the time window used in the regression analysis. It is recommended that at least a 3-year time span should be used for deriving the average subsidence rate from the PPP solutions of continuous GPS sites. A longer time period may be required when invoking OPUS solutions and/or the campaign GPS survey. It is advisable to use both the OPUS and PPP methods as a quality assurance check for critical positional and velocity measurements.