The pilot had been conducting a six and a half hour low level survey flight. He stated that on joining the circuit, at the completion of the flight, he lowered the landing gear and obtained a gear down indication. The gear indication was again checked on final approach but during the subsequent landing roll, as the nosewheel was being lowered to the runway, the landing gear handle in the cockpit sprung to the up position. The landing gear retracted and the aircraft slide to a halt on its undersurface. An inspection of the aircraft could find no pre-impact defect with the landing gear or its systems which could have contributed to this inadvertent retraction. The landing gear was found to be capable of normal operation. The reason for the selector deselection remains undetermined.