The ColFFTWG-DTU2019 gravimetric quasi-geoid model has been computed by the Technical University of Denmark (DTU), Copenhagen. 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 250°E ≤ longitude ≤ 258°E, 35°N ≤ latitude ≤ 40°N with a grid spacing of 1' in both latitude and in longitude. The computation was performed by the GRAVSOFT package in the framework of a remove-compute-restore procedure. XGM2016 was used up to degree and order 360 as global gravity model, and SRTM v4.1 was used for the residual terrain reductions at 3″ resolution for both airborne and terrestrial data, computed from a 9″ mean model. The linear Prey term approximation was applied for the harmonic correction of surface gravity points below the mean elevation surface (Forsberg and Tscherning, 1981). The reduced airborne gravity data were downward continued and gridded, together with surface gravimetry, at the terrain surface using 3D Least-Squares Collocation with planar logarithmic covariance function (Forsberg, 1987). The resulting 1'x1' grid was converted to a residual quasi-geoid by spherical FFT, using a Wong-Gore modified Stokes kernel, with a low-wavelength cut-off transition band at harmonic degrees 180-190, to keep the GOCE information unchanged up to this degree. Residual terrain effects and the XGM2016 contribution were restored to obtain the final gravimetric quasi-geoid. The accuracy of the quasi-geoid model, when compared against GSVS17 GPS/leveling, is equal to 3.1 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).