Current aircraft noise models, including USAF's NOISEMAP and FAA's Integrated Noise Model (INM), assume that surrounding terrain is flat and has nominal impedance properties. A study has been performed to evaluate the significance of elevation and ground impedance variations on predictions, and also the practicality of including these effects in current computer models. Emphasis is on NOISEMAP, ... [Show full abstract] but the conclusions are generally applicable to the INM as well. Slight differences will occur between the two models due to differences in their current lateral attenuation algorithms. Elements addressed were the availability of suitable theory for the physical effects involved, availability of ground property data, the effect on program execution time, and the magnitude of the difference in sound level.