An instructor who was watching the aircraft as it entered the circuit estimated that the aircraft was about 200 feet too low on the downwind leg. The base turn was conducted at about 50 feet and during the turn onto final the wing of the glider struck the boundary fence. A subsequent examination indicated that the altimeter was over-reading by some 200 feet. The altimeter had been set to indicate zero prior to departure. In the following four and a half hours pressure changes had occurred which resulted in the apparent over-reading. During the circuit, the pilot had been concerned with other traffic about to take-off, and had attempted to alter the flap setting in order to modify the circuit. However, he had inadvertently applied the dive brakes and the glider lost height rapidly until the situation was corrected. A landing straight ahead was considered, but the pilot elected to try and reach the strip. It was likely that he was suffering a degree of fatigue after a long flight.