Performance of Network-Based RTK GNSS for the Cadastral Survey in Thailand

Abstract

In Thailand, a network of GNSS Continuously Operating Reference Station (CORS) has been operating as a Network-based Real Time Kinematic (NRTK) GNSS positioning solutions since 2008.  The Department of Lands designs a Thai-NRTK system mainly for GNSS cadastral survey using the Virtual Reference Station (VRS) as its positioning concept.  The current Thai-NRTK consists of 114 GNSS CORSs covering only some parts of Thailand.  The CORS spacing ranges from 25.1 km to198.6 km.  According to previous studies, the performance of NRTK-GPS positioning determination in Thailand is strongly depending on the dimension of the triangular CORS locations; known as a “loop”. This research aims to examine the horizontal position performance of this Thai-NRTK GNSS positioning solutions as a quality indicator for GNSS cadastral surveying.  The test was carried out with quite a large number of test samples (total of 2,122 points) and where every loop is separated into 4 test groups with differing the station distances starting from the loop spacing of: 30 km to 50 km, 50 km to 70 km, 70 km to 90 km and 90 km to 110 km, respectively.  The positioning performance test is conducted in two main folds; namely, positioning accuracy and time to resolve its solutions.  On positioning accuracy, the test results show that the spacing of every tested loop achieves the accuracy in accordance with the cadastral surveying requirements.  This proves that the Thai-NRTK GNSS positioning network and technique is well-designed and suitable for cadastral positioning within its coverage areas.  On time to resolve fixed solution, the test results indicate that the ‘time to first fixed’ and the ‘rate of the horizontal position jump’ increase with longer loop distance; whereby only loop length of 30 km to 50 km has 100% success ‘rate of ambiguity-fixing’ solutions; therefore, in order to achieve a high reliable performance positioning from the Thai-NRTK GNSS solutions for cadastral survey in Thailand, it is recommended that the reference stations shall be kept within 50 km.