Agricultural positioning systems must be reliable, cost-effective and should have a precision according to the field operation to be carried out. The Global Positioning System (GPS) gives the opportunity to use commercially available receivers. Since dynamic real-time positions are needed for farming, reflection or absorption of satellite signals due to buildings, hills, trees, etc., should not disturb the position determination. The coupling of independent location information can be used to provide positions and updates through the down time. The field geometry (borders, direction and distance of rows), stored in an electronic map, is utilized with an internal system (speed and direction) to provide the positions when the GPS signal is disrupted. The combination of these various information sources (geometry, internal system, GPS) gives a way of increasing the reliability and accuracy of the whole system. Three different possibilities for the determination of the direction have been tested: (i) difference of two parallel radar sensors, (ii) fluxgate compass, and (iii) piezoelectric vibratory gyroscope. The experimental results and the principal algorithm for the on-line coupling of GPS, sensor data, and the field map are presented.