A control system for controlling positioning of a header mounted to a main body of an agricultural harvester, the header being pivotable by an actuator about a central axis of the main body, the control system comprising: at least three header height sensors (52, 54, 56) comprising: a first header height sensor for mounting at a first point on the header, and configured to provide a first header height signal representing a measured first header height (H L) of the first point on the header above a ground plane; a second header height sensor for mounting at a second point on the header, and configured to provide a second header height signal representing a measured second header height (H R) of the second point on the header above the ground plane; and a third header height sensor for mounting at an intermediate point on the header between the first point and the second point, and configured to provide a third header height signal representing a measured third header height (He) of the intermediate point above the ground plane; and a header angle sensor configured to provide a header angle signal indicative of a current pivot angle (θ 1) of the header about the central axis; characterized in that the control system further comprises a processor configured to: receive the signals provided by the header angle sensor and the at least three header height sensors (52, 54, 56); calculate an estimated first header height based on the current pivot angle (θ 1) and the measured second and third header height (H R, He); determine a replacement first header height by selecting the smallest of the estimated first header height and the measured first header height (H L); and generate a control signal for positioning the header, comprising controlling the actuator to pivot the header based at least on the replacement first header height. at least three header height sensors (52, 54, 56) comprising: a first header height sensor for mounting at a first point on the header, and configured to provide a first header height signal representing a measured first header height (H L) of the first point on the header above a ground plane; a second header height sensor for mounting at a second point on the header, and configured to provide a second header height signal representing a measured second header height (H R) of the second point on the header above the ground plane; and a third header height sensor for mounting at an intermediate point on the header between the first point and the second point, and configured to provide a third header height signal representing a measured third header height (He) of the intermediate point above the ground plane; and a first header height sensor for mounting at a first point on the header, and configured to provide a first header height signal representing a measured first header height (H L) of the first point on the header above a ground plane; a second header height sensor for mounting at a second point on the header, and configured to provide a second header height signal representing a measured second header height (H R) of the second point on the header above the ground plane; and a third header height sensor for mounting at an intermediate point on the header between the first point and the second point, and configured to provide a third header height signal representing a measured third header height (He) of the intermediate point above the ground plane; and a header angle sensor configured to provide a header angle signal indicative of a current pivot angle (θ 1) of the header about the central axis; characterized in that the control system further comprises a processor configured to: receive the signals provided by the header angle sensor and the at least three header height sensors (52, 54, 56); calculate an estimated first header height based on the current pivot angle (θ 1) and the measured second and third header height (H R, He); determine a replacement first header height by selecting the smallest of the estimated first header height and the measured first header height (H L); and generate a control signal for positioning the header, comprising controlling the actuator to pivot the header based at least on the replacement first header height. receive the signals provided by the header angle sensor and the at least three header height sensors (52, 54, 56); calculate an estimated first header height based on the current pivot angle (θ 1) and the measured second and third header height (H R, He); determine a replacement first header height by selecting the smallest of the estimated first header height and the measured first header height (H L); and generate a control signal for positioning the header, comprising controlling the actuator to pivot the header based at least on the replacement first header height.