Estimation of Local Subsidence Using GPS and Leveling Combined Data

The leveling measurements have been traditionally made for the monitoring of ground subsidence, but this technique is known to be not timeand cost-effective. Instead, GPS has been recently used as one of the most convenient and precise techniques for the geodetic applications, in terms of high accuracy monitoring. A GPS monitoring network consisting of 52 GPS stations was set up in Yunlin County, the southwestern part of Taiwan, in order to determine the first epoch data set and investigate the feasibility of monitoring the local ground subsidence with the use of GPS. The leveling-derived heights were still required for the estimation of ground subsidence using the integrated models, as the GPS-derived heights based on a long term of observation have not been completely established. It has been found from the estimates that the geoidal height, a parameter of datum between the two height systems, has significant influence on the reliability of subsidence estimated using the integrated models with combined data. The RMS difference of up to 13 cm can be found between the annual subsidence rates estimated by using the so-called ‘absolute’ and ‘relative’ modes of integrated model for the study area. However, the ‘relative’ estimates of the subsidence, in which a differential mode of geoidal information between the two neighboring sites was used, were tested to be consistent with those using two sets of GPS solutions by a RMS difference of around 5 cm.