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About changing Sea Levels

The heights of mountains and the altitude of aircraft
are often given as ‘above sea level’, as if the level of the sea
was permanently fixed. Yet even in the present day, it is very
difficult to pinpoint the level of the sea accurately. The sea moves
continuously, not only with the tides, but with wind, waves, and
currents, and even daily changes in temperature and chemistry. In
the UK, heights are measured relative to the Ordnance Datum, an
average achieved by year upon year of measurements of the mean
sea level shown by tide gauges at particular sites.

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