The ColLSMSA-KTH2019 gravimetric quasi-geoid model has been computed by the University of Gävle, the Lantmäteriet and the Royal Institute of Technology (KTH) in Sweden. The model 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 251°E ≤ longitude ≤ 257°E, 36°N ≤ latitude ≤ 39°N with a grid spacing of 1' in both latitude and in longitude.

The quasi-geoid is computed using a two-step procedure. First, the terrestrial and de-biased airborne gravity anomalies are gridded using a Remove-Compute-Restore technique and three-dimensional Least Squares Collocation (LSC) with spherical Tscherning and Rapp (1974) type of covariance functions. This step achieves downward continuation of the airborne gravity data and combination with the terrestrial observations. In the second step, the resulting surface gravity anomaly grid is used to compute height anomalies by using Least Squares Modification of Stokes’ formula with Additive corrections (LSMSA or KTH method). The GEOID17RefB global gravity model up to degree 2190 is used in the first gridding step, while the satellite-only GOCO05S model up to degree 240 is used in the second step. The accuracy of the quasi-geoid model, when compared against GSVS17 GPS/leveling, is equal to 2.8 cm.

The quasi-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).