Abstract: In a highway survey vehicle, grade and cross fall measurements are made in relation to references established by a displacement type pitch and roll gyroscope with a gravity referenced erection system. Lateral acceleration of the vehicle is calculated independently and used to provide a correction for errors caused by spurious erection induced by such lateral acceleration, provided that such acceleration falls within certain limits such as to permit effective correction. The lateral acceleration may be calculated from measured rates of displacement and change of the direction of the vehicle.