The plane one-dimensional photon transport equation is solved for the scattered γ-radiation flux in the case of two adjacent media. One medium represents a natural ground with uniformly distributed potassium, uranium, and thorium γ-ray emitters. The other medium is air with no radioactive contaminants. The solution method is the double-P1 approximation with equidistant lethargy mesh. By this approach, which is discussed in some detail, it is possible to calculate the complete terrestrial γ-radiation field in an economical way. Calculated ground-level γ-ray exposure rates for igneous rocks with effective atomic numbers in the range 13.1 to 15.5 are presented.