An enhanced vehicle guidance system comprising a global navigation satellite system (GNSS) receiver and a data processor with a memory component and a computing device. The method of enhancing a vehicle's guidance system may comprise calculating the altitude, latitude, and longitude of a GNSS receiver for each of a plurality of positions; calculating the incline angle between adjacent points; and using the calculated incline angles to infer the attitude of the vehicle at any of the plurality of positions. The attitude may be used to calculated an inertial correction factor to compensate for GNSS position inaccuracies induced as a result of the vehicle rolling and pitching on uneven terrain. The altitude, latitude, longitude, and attitude of the plurality of positions may be stored in the memory such that the system may look-up the attitude for a given position without recalculating the attitude and without using an inertial sensor.