Uncertainties related to in situ gamma-ray spectrometry measurements have been widely acknowledged. Despite being commonly used in exploration, the evaluation of the uncertainty range of K, U and Th gamma spectrometry field measurements is generally limited due to the lack of corresponding laboratory measurements. Estimating uncertainty, however, is essential to ensure reliable and accurate results.In this paper, we establish a set of procedures to acquire gamma-ray spectrometry data while minimizing uncertainty, based on high-resolution lithostratigraphy and composite gamma-ray logging techniques. We compare the concentration of radionuclides measured in situ in 250 selected sites in outcrops of the Lower and Middle Jurassic sedimentary successions of the Lusitanian Basin to the concentration of radionuclides in representative samples collected from the same outcrops and analysed in the laboratory.The global radiometric dataset obtained for K, U and Th is similar between field and laboratory gamma spectrometry measurements, which are correlated. The root-mean-square deviation was determined for K, U and Th, and the error of the field measurements was estimated at 0.6%, 1.4 and 2.5 ppm, respectively. We calculated the relative error (RE) between laboratory and field data. If we use the interquartile range of the RE to distinguish outliers in each radionuclide dataset, approximately 8%, 6% and 10% of the K, U and Th field measurements correspond to outliers. Conversely, 92%, 94% and 90% of the samples offer reliable field data for K, U and Th, respectively. The RE is similar between K and U, with a mean of 0.73 and 0.69, respectively, and is higher for Th (with a mean of 1.29). Higher RE values are also observed in dolostones (D), dolomitic limestones (DL), limestones (L) and marly limestones (ML). Lower RE values are linked to sandstones (S) and fine-grained siliciclastic (F-g.S) lithologies, with a mean value of 0.19, 0.33 and 0.25 for K, U and Th, and 100% of reliable data.As the accuracy of field gamma-ray measurements varies with lithology, field procedures must be adjusted for lithological variability to reduce uncertainty. Given that the RE decreases with an increase in layer thickness, thicker sedimentary layers should be favoured when measuring K, U and Th concentration in field campaigns.