Following a winch launch, the pilot spent 12 to 15 minutes gliding before returning for a landing. On the downwind leg he noted that the aircraft appeared to be lower than the height indicated on the altimeter. At about the base leg position the aircraft was very low, and witnesses expected the glider to land in one of several suitable paddocks. However, the pilot continued towards the strip and the glider touched down during the turn onto final approach. The tail section was broken off when it contacted long grass. The pilot had accumulated most of his gliding experience at the particular strip and was familiar with the area. The flight in question was to be the first made by the aircraft since returning from another aerodrome. During his pre-launch checks the pilot had forgotten to re-set the altimeter to read zero feet. As a result, the altimeter was over-reading by some 500 feet. The pilot had concentrated on the indicated height and had not visually assessed the approach profile. He was unable to explain why he had persisted with the approach when he became aware that the aircraft was abnormally low and there were suitable outlanding areas available.