Gamma rays can penetrate several hundred metres of air, but only 30-35 cm of rock or soil. Therefore, most gamma-rays measured above the Earth's surface originate in the top 35 cm of the earth's surface. The interpretation of gamma-ray data requires an understanding of surface processes such as weathering and the surficial transport of regolith material. Interpreters also need to be aware that the radiation measured by an airborne spectrometer at 60-100 m above the ground represents the average radioactivity over a large area (Figure 3). Also, be aware that disequilibrium is common in the U decay series. High eU estimates may well represent Ra concentrations rather than that of 238U.