1. An efficient method for determining from a large plurality radio transmitters within vicinity of a radio receiver, the radio transmitters with signals that could potentially interfere with a signal intended to be received at the radio receiver , said method consisting of the steps of: dividing a geographic area into smaller sized geographic blocks; determining a roughness factor for each of said blocks; determining an average terrain elevation for each of said blocks; storing said average terrain elevation and said roughness factor for each of said geographic blocks in a database; identifying geographic coordinates of said radio receiver under study; identifying geographic coordinates of one of said plurality of radio transmitters; identifying the geographic blocks containing said coordinates for both said receiver and said one of said said transmitters and any geographic blocks located between said receiver and said one of said transmitters; recalling from said database said average terrain elevations and said roughness factors for said identified geographic blocks; calculating a signal power level received at said receiver from said transmitter using a smooth terrain propagation model if the average terrain elevation for said identified blocks is below a predetermined threshold; calculating a signal power level received at said receiver from said transmitter using a rough terrain propagation model if the average terrain elevation for said identified blocks is equal to or above said predetermined threshold and using the lowest block roughness factor from said identified geographic blocks in place of the actual path terrain information in the loss calculations; and identifying a transmitter as a potentially interfering transmitter if said calculated signal power is above the allowed signal power for an interfering signal allowable for said receiver.