Most of the energy detected during these flights was from rocks and soils within 800 feet of flight lines that were spaced 1 to 6 miles apart. Many major metropolitan areas were not covered by the survey because of flight restrictions. Therefore, only a small part of the surface of the United States was measured. The data from this survey, however, give a good indication of the background uranium concentration of soils and rocks underlying most of the United States. The smallest data point on the map covers about 1.6 by 1.6 miles, limiting the amount of detail that can be seen.
Scientists also measure radon in soil air.
These data give direct evidence about soil radon, but extensive sets of these data are not commonly available. The two basic methods for measuring the radon concentration of soil air are the same as those used to measure radon in buildings. Both methods measure the alpha particles produced by the decay of the radon in the air. One method involves burying a passive device, such as a charcoal cannister or an alpha-track detector, in the soil and leaving it open to the soil air. This method allows long-term measurements, but the devices can be affected strongly by soil moisture.