Atmospheric delays sometimes cause non-negligible errors in the measurement of ground surface displacements using GPS, especially when there is a large difference in height between a measurement point and the reference point of the GPS receiver. In the previous studies, the authors have suggested a correction method for such atmospheric delays, which uses a modified Hopfield model and on-site hourly-observed surface weather data at site. In order to evaluate the effectiveness of this correction method, long-term field measurements were conducted in the present study on a large slope in an open-pit limestone quarry. Nine GPS receivers were set on the slope as the measurement points with height differences of 106 ∼ 209 m to a fixed reference point, and their three-dimensional displacements were measured every hour. The measurement results from the two-year period revealed that the conventionally processed GPS displacements at all measurement points clearly showed yearly-periodic variations and short-wave irregularity in the vertical component, which were concluded to be due to atmospheric delays caused by the difference in height between the measurement points and the fixed reference point. On the other hand, applying our suggested atmospheric delay correction method, we confirmed that the correction could successfully remove the yearly-periodic variations and short-wave irregularity. These results prove that the GPS ground surface displacement measurement system is also applicable to sites where there is a large difference in height of over 100 m, if the atmospheric delays are corrected using the modified Hopfield model and on-site surface weather data.
