Large-scale airborne geophysical data are an important part of regional-scale mineral exploration and in that context NGU conducted an airborne magnetic and radiometric survey over the Kviteseid, Notodden and Ulefoss regions, in Telemark county in summer 2013 as a part of the MINS project (Mineral resources in South Norway). Results from a previous high resolution helicopter-borne gamma-ray spectrometry survey that was carried out in 2006 over the Fen Complex, Ulefoss region, were reevaluated in the light of the new data from the regional survey of the MINS project. We compare the thorium (eTh) ground concentrations of the two surveys, and although they have similar pattern, are showing significant scaling difference. The differences in the level of ground concentrations between two spectrometry surveys flown over the same region were investigated. Reprocessing of the old data revealed that the discrepancies arose from inappropriate selection of sensitivity coefficients during the calculation of elemental ground concentrations from height corrected gamma-ray counts. Therefore, correct calibration of the spectrometers are essential for the quality of the processed data.