Geographic Information System (GIS) technology has made it possible to compare indoor radon measurements at almost 2000 precisely located homes in northern Virginia with the total gamma aeroradioactivity at each site, along with many other geologic and topographic site characteristics. Total gamma radioactivity, derived mainly from radioactive isotopes of potassium and uranium series elements (which includes radon) tend to increase together. Consequently, total-gamma aeroradioactivity tends to be greater in areas with greater radon emanation from the soil, and greater in areas around homes with greater indoor radon. Our comparison of total-gamma aeroradioactivity with indoor radon in homes built on the soils of nine different geologic units all show that indoor radon increases as total-gamma aeroradioactivity increases.The correlation is better than other indicators, such as the average soil radon concentration over particular geological units and the age of the home.