The ColUNBSH-GSI2019 gravimetric quasi-geoid model has been computed by the Geospatial Information Authority of Japan (GSI) in cooperation with the Technical University of Denmark (DTU). It has been worked out in the frame of the International Association of Geodesy Joint Working Group 2.2.2 "The 1 cm geoid experiment" and the so called "Colorado experiment". The area covered by the model is 109°W ≤ longitude ≤ 103°W, 36°N ≤ latitude ≤ 39°N with a grid spacing of 1' in both latitude and in longitude. Terrestrial and airborne gravity data are combined and gridded by least-squares collocation with the planar logarithmic covariance model. The resulting grid of free-air gravity anomalies is converted into Helmert gravity anomalies by applying the spherical topographic reduction and the analytical downward continuation with Helmert's second method of condensation. The computation method is based on the remove-compute-restore technique, using XGM2016 up to degree and order 719 as the reference global gravity model. In particular, the geoid model is computed by Stokes integration of residual Helmert gravity anomalies with the hybrid Meissl-Molodensky modified spheroidal Stokes kernel (UNB Stokes-Helmert scheme). Finally, the quasi-geoid model is obtained by subtracting the classical geoid/quasi-geoid separation term from the geoid model. The accuracy of the quasi-geoid model, when compared against GSVS17 GPS/leveling, is equal to 2.9 cm.

The geoid model is provided in ISG format 2.0 (ISG Format Specifications), while the file in its original data format is available at the model ISG webpage.

The International Service for the Geoid (ISG) was founded in 1992 (as International Geoid Service - IGeS) and it is now an official service of the International Association of Geodesy (IAG), under the umbrella of the International Gravity Field Service (IGFS). The main activities of ISG consist in collecting, analysing and redistributing local and regional geoid models, as well as organizing international schools on the geoid determination (Reguzzoni et al., 2021).