The ColFFTDB-CGS2019 gravimetric quasi-geoid model has been computed by the Canadian Geodetic Survey (CGS), Natural Resources Canada. 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 110°W ≤ longitude ≤ 102°W, 35°N ≤ latitude ≤ 40°N with a grid spacing of 1' in both latitude and in longitude. The solution is based on the Stokes-Helmert method, which is used with the degree-banded Stokes kernel modification (Huang and Véronneau, 2013). A band-wise spectral scheme is developed to combine the global model, airborne and surface gravity data, contributing to low-, medium- and high-degree bands of the geoid, respectively. In particular, the global gravity model provides entirely the low-degree band up to degree 150 from where it goes into transition with the surface gravity data up to degree 210. The surface gravity data complete the geoid frequencies up to degree 10800 with complement information from SRTM 3″ digital terrain model. As for the GRAV-D airborne gravity data, they provide corrective values directly to the geoid model, contributing between degrees 210 and 790 with smooth transition at each end. The geoid computation is performed by Fast Fourier Transform (FFT) numerical integration and then the quasi-geoid model is obtained by adding a classic geoid to quasi-geoid correction grid. The accuracy of the quasi-geoid model, when compared against GSVS17 GPS/leveling, is equal to 1.8 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).
