I WENT to Newlyn, Cornwall, to see the Primary Benchmark of British Mean Sea Level, a large bronze bolt in the harbour. For some reason I expected it to be at, or at least near, sea level. I thought I would just have to peer over the harbour wall, and there it would be. But, unfortunately, it wasn't that simple. The benchmark has to be accessible to surveyors. But it has to be protected.

I walked along the south pier of the harbour to the tidal observatory, a small concrete hut with a steel door, reinforced against rough seas. The observatory (right) is part of the national tide gauge network, which warns of coastal flooding. An engineer was working inside. He showed me the bolt, set into the floor (above), beneath a protective cover.

'Not many people have seen this,' he said. Newlyn was chosen as the site for the benchmark because the Atlantic Ocean is scientifically most reliable in this area, compared with other parts of the coast. Between 1 May 1915 and 30 April 1921, readings of the sea level were taken every hour. In this way mean sea level was established. It is the official basis for height calculation on British maps. The bolt, however, is only a reference point: mean sea level (the 1915-21 average) is 4.751m below.

The engineer replaced the cover. 'Every height around the UK is related to the top of that bolt,' he said. 'It's nice to know where you stand,' I remarked. He smiled faintly. I left him working among his tide gauges, and walked back round the harbour towards Penzance. I noticed that the sea had fallen slightly.

The Primary Benchmark of British Mean Sea Level is at Ordnance Survey grid reference SW 46762855.

(Photograph omitted)