The DRUKGEOID2015 model is a geoid of Bhutan, covering the area from 88.5°E to 92.5°E in longitude and from 26.5°N to 28.5°N with a grid resolution of 0.02°. It is referred to the GRS80 ellipsoid and it is based on a set of 255 points with both GNSS and gravity, collected in the framework of this project. Gravity observations were observed by the National Land Commission (NCL) using two CG5 gravimeters. As for the GNSS observations, the positions were computed using the differential GNSS approach for points in the proximity of Permanent Reference Stations (PRS). The Precise Point Positioning (PPP) technique were employed. The DRUKGEOID2015 model was computed by applying the remove-restore procedure, where the used global geopotential model was EIGEN-6C4 and the topography information was taken from the Shuttle Radar Topography Mission (SRTM) model. After the removal step, gravity residual were converted into residual height anomalies using the Least-Squares Collocation (LSC) approach. After the restoring step, height anomalies were converted into geoid undulations using the algorithm by Flury and Rummel (2009) and the SRTM topography. It was decided to fix the resulting gravimetric geoid to the fundamental benchmark at NLC headquarters in Thimphu (TH01). At this point the GNSS ellipsoidal height with respect to the DRUKREF03 system (ITRF2000 at epoch 2003.87) minus the geoid undulation is equal to the official orthometric height. Comparison of 27 GNSS/levelling benchmarks with DRUKGEOID15 showed a standard deviation of 0.55 m.
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).
