2018 Volume 64 Pages 14-25
The Geospatial Information Authority of Japan (GSI) has carried out nationwide gravity measurements covering the whole of Japan for about 60 years and provided a nationwide gravity standard network. It has been widely used for various purposes such as calculation of orthometric heights, calibration of weighing instruments and exploration of underground structure. However, modernization of the gravity standard network is required because of gravity changes caused by crustal deformation and it has become possible to be realized thanks to recent improvements of gravimeters. Therefore GSI released the modernized new gravity network, named the Japan Gravity Standardization Net 2016 (JGSN2016), on 15 March 2017 for the first time in 40 years.
JGSN2016 consists of gravity values at 32 fundamental gravity points and 231 primary gravity points. Each gravity value was calculated by following steps; 1) determining absolute gravity value of each fundamental gravity point based on absolute gravity measurements, 2) measuring gravitational differences along each baseline by relative gravity measurements and 3) calculating the most probable values of each primary gravity point by carrying out network adjustment with fixing the values of 1). In the course of the above procedures, we corrected for the influences of solid earth tide, atmospheric pressure, polar motion and ocean tide after removing outliers by statistical screening. In addition, the most appropriate combination of parameters for the network adjustment was determined by using the Bayesian Information Criterion (BIC). The absolute gravimeters were calibrated through comparison with the one owned by the National Institute of Advanced Industrial Science and Technology (AIST) who periodically joins in the recent International Comparison of Absolute Gravimeters (ICAG). Thanks to all these kinds of improvements, JGSN2016 achieves 3.0 μGal and 11 μGal precision for the fundamental and primary gravity points, respectively.
The final products of JGSN2016 are freely provided from GSIʼs website (https://sokuseikagis1.gsi.go.jp/top.html) and all data of the absolute gravity measurements are registered to the Absolute Gravity Database (AGrav) with expecting to contribute to activities of Global Geodetic Observing System (GGOS). This paper reviews the details of JGSN2016 construction with briefly touching the history and significance of GSIʼs gravity measurements.
