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Cartopics > How long is the UK coastline?

The true answer is: it depends! It depends on the scale at which you measure it.

What do we mean by that? Well, if you got a small-scale map (1:2,500,000 — for example an atlas where the whole of Great Britain is shown on one page) and lay a piece of string as carefully as you could on the coastline, you could then measure the length of the string and work out how long the coast is, knowing the scale of the map. However, if you then take an atlas at a larger scale (for example a road atlas at 1:250,000) and do the same exercise, you would find that the coastline is longer than it was when you measured from the atlas page. If you repeat the exercise with a series of much larger scale maps (say 1:10,000), you would find that the answer is greater still.

Why is this? Well, it relates to the ‘wiggliness’ of the coastline. The larger the scale of map, the more detail (and so the more coastal wiggles — inlets, headlands, etc.), it will show. If you took a measuring ruler out onto the beach and tried to measure the coastline, with the sea perfectly still, you would find that in laying down the ruler where the water meets the land you will see wiggles either side of the ruler, showing that the coastline must be longer than the ruler. If you tried a shorter ruler, you’d find that the coast still wiggled around the ruler, and will continue to do so however small a ruler you use. This is an example of fractal behaviour, and the result is that the only truly ‘accurate’ measure of the length of the coastline — any coastline — shows that it is almost infinitely long.

So, any figure for the length of a coastline depends on the scale of the map that has been used to measure it, and that’s why figures quoted for the length of the coastline vary quite a lot.

Perhaps that’s not very helpful! You want a figure you can quote, so here you are: according to Ordnance Survey, the length of the coastline of Great Britain (the main island only) is 11,072.76 miles (17,819.88 kms). This figure is derived from 1:10,000 maps, and represents the length of the mean high water mark. It is the result of using a computer to measure the length of the mean high water line on a digital map. It’s somewhat misleading to quote the figure to two decimal places (in other words 10m precision) because the length of the coastline will vary as the shoreline evolves, with some beaches disappearing and others getting larger on a continual basis. It would be more reasonable to say that it is about 11,073 miles or about 17,820 kms. The AA also quotes the figures of 11,073 miles (17,820km).

The following figures are lengths measured from maps along the mean high water mark:
  • The length of coastline of Great Britain plus its principal islands is about 19,491 miles (31,368 km).

  • The length of coastline of England only is about 5581 miles (8982 kms), and of mainland England plus the Isle of Wight, Lundy and the Scilly Isles is 6261 miles (10,077 kms)

  • Mainland Wales’s coastline is about 1317 miles (2120 kms) and adding the islands of Anglesy, and Holyhead the figure is 1680 miles (2740 kms)

  • Mainland Scotland measures about 4174 miles (6718 kms) but adding the islands of Arran, Islay and Jura, Shetland and Orkney, and the Western Isles brings the total to 11,550 miles (18,588 kms).

Giles Darkes
January 2008

Acknowledgements: Ordnance Survey and David Irvine of the AA.
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