These average radon values were produced by the high radon project of Lawrence Berkley National Laboratory from a semi-empirical model that used short and long-term radon measurements and surface radon progeny concentrations. For details see http://www.stat.columbia.edu/radon/ .
This map was provided by Dr. Dan Steck and his students from “The Minnesota Radon Project.” They are located in the Physics Department at St. John’s University in Collegeville, Minnesota.