Testing Horizontal Coordinate Correction Model Used for Transformation from PPP GNSS Technique to Thai GNSS CORS Network Based on ITRF2014

Abstract

The movement of tectonic plates directly affects the coordinates of geolocation and reference frame at the same position over time. Many international organizations and agencies have attempted to improve the reference frame to be more consistent with the current plate movements. In this study, a horizontal coordinates correction model based on the International Terrestrial Reference Frame 2014 (ITRF2014) in Thailand is tested by applying the Precise Point Positioning (PPP) technique computed by the GipsyX software. Coordinates of the points from some Continuously Operating Reference Stations (CORS) distributed over the country are used in the test. These points are observed twice.  They are divided into two groups: training points and check points. GNSS observations from these points are processed with the PPP techniques to get 3D coordinates of all the points. The coordinates of the training point are used to compute the shift rates between the two epochs. Then, the surfaces are constructed by interpolation of the shift rates at the training points. The testing is based on four types of the interpolation: Inverse Distance Weighted (IDW), Kriging, Natural Neighbor, and Spline. In addition, vertical values ​​often have problems with different types and heights of various kinds of antennas from the CORS stations and vertical values are mainly based on field work form leveling survey. This paper will focus on only the horizontal coordinates for grid transformation. Each of the surfaces is sampled into a grid which is used to compute correction for the coordinates of the check points. The differences between the two versions of the coordinates of check points (the coordinates directly obtained from observations and the coordinates, from the other epoch, corrected to the same epoch as the former) becomes errors. The root mean square errors (RMSEs) of horizontal coordinates are computed based on 149 checkpoints scattered throughout the country. The findings indicates that the correction models of IDW, Kriging, Natural Neighbor, and Spline method give horizontal coordinate accuracies of 0.011, 0.010, 0.017, and 0.017 meters, respectively. Thus, this can improve the accuracy of horizontal coordinates for the ITRF2014 in Thailand to less than 2 cm. The result is equivalent to less than 3 cm and 4 cm for the confidence level of 95% and 99.7%, respectively.