The European Atlas of Natural Radiation developed by the Joint Research Centre (JRC) of the European Commission includes maps of potassium K and thorium Th. With several different databases available, including data (albeit not calibrated) from an airborne survey, Belgium is a favourable case for exploring the methodology of mapping for these natural radionuclides. Harmonized databases of potassium and thorium in soil were built by radiological (not airborne) and geochemical data. Using this harmonized database it was possible to calibrate the data from the airborne survey. Several methods were used to perform spatial interpolation and to smooth the data: moving average (MA) without constraint, or constrained by soil class and by geological unit. Overall, there was a reasonable agreement between the maps on a 1 × 1 km2 grid obtained with the two datasets (airborne data and harmonized soil data) with all the methods. The agreement was better when the maps are reduced to a 10 km × 10 km grid used for the European Atlas of Natural Radiation. The best agreement was observed with the MA constrained by geological unit. ; JRC.G.10 - Knowledge for Nuclear Security and Safety