Seeing sea level

07/08/2007 by Lee.

Most of us have been on a plane at some point in our lives and when flying the altitude of the aircraft is generally given as x feet above sea level. The reasons for this are quite obvious when you consider that land can form high plateaus or low valleys and using the simple measure of height over the land could lead to disastorous consequences when flying from Schipol (below sea level) to Bolivia.

Tidal patterns lead to differing sea levels around the world at different times. For example around Alaska and parts of Canada the tide can lead to a change in sea level of over 50 feet. Whilst this may not sound like much, imagine trying to land a plane in bad weather and poor visibility and suddenly finding the runway 50 feet away just as you cut the power. A change in air pressure can give rise (or indeed fall) to a change in sea level of another 5 feet or so adding to the height confusion.

It’s not just aircraft that suffer from rising and falling sea levels. Mountains are also measured in height above sea level, so depending on the tide in a given part of the world could one summit attempt on Everest be said to have been higher than another? Since the last ice age, sea levels have risen by around 400 feet thereby shrinking many of our mountain ranges. Now, whilst we can safely say that they haven’t actually shrunk (other than by normal erosion and tectonic methods) the standard measure that we use indicates that they have.

So what is the standard or “mean” sea level? In short, there isn’t one. In the U.K. sea level is determined by a guage just outside Newlyn in Cornwall, yet this is over a foot different to sea level on the east coast of the island. GPS systems use a complex ellipsoid model of the Earth to work out a mean sea level, but neither of these methods can give us 100% accuracy.

So throughout the course of today, your altitude will vary depending on which method you use, and if you’re flying anywhere shortly don’t do the Amsterdam to La Paz route.

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