A machine control system and method includes an on-board GPS receiver, an on-board processor adapted to store a preplanned guide pattern and a guidance device. The processor includes a comparison function for comparing the vehicle GPS position with a line segment of the preplanned guide pattern. The processor controls the guidance device for guiding the vehicle along the line segment. Various guide pattern modification functions are programmed into the processor, including best-fit polynomial correction, spline correction, turn-flattening to accommodate minimum vehicle turning radii and automatic end-of-swath keyhole turning.