25
Jan
2017
0

Which English county has the longest coastline?

Our Media team were recently asked to confirm whether Essex was the English county with the longest coastline. That should be easy enough, right? We have some very talented geographic information (GI) analysts at OS and a database containing over 450 million features across Great Britain. But it’s not actually that simple. The length of the coastline can be a very contentious fact. Here’s why.

Essex

Firstly, the length of the coastline changes on a daily basis. With changing tides across the days and during the seasons, we get a higher tide or a lower tidal point – which affects any measurement on the length of coastline.

Secondly, the length of the coastline increases the more detailed the mapping. The more detailed the mapping, the more coves and nooks and crannies that are being measured. As an example, the figure for mapping the length of Cornwall’s coastline increases by 100 km between using 1:50,000 scale maps (OS Landranger map) and 1:10,000 scale mapping.

How do we measure the length of the coastline at OS?

We used our Boundary-Line product, an open dataset, which is at 1:10,000 scale and is the scale also used by the Boundary Commission. Comparing the length of English county coastlines using Boundary-Line high-water data, Essex didn’t top the table. Cornwall has the longest coastline (even without including the Isles of Scilly).

  1. Cornwall: 1086 km
  2. Essex: 905 km
  3. Devon: 819 km

Cornwall

So, for anyone planning to walk or swim the coastline, that would give you a rough idea of the distance you’re covering.

For a brilliant and detailed explanation about the differing lengths of Great Britain’s coastline (which we calculate to be 17,820 km using the same method as above), check out this blog by Alasdair Rae.

For more on calculating sea level and how this affects the height of mountains, read our blog by geodesist Mark Greaves.

3 Responses

  1. Steve Edge

    Of course, it rather depends on what you define as “coastline”. Using MHW is not necessarily the right datum (although I can see why the OS might prefer to use it).

    Many years ago, I used OS MHW data to compute the length of “coastline” of English counties and rather surprised to find Nottinghamshire (which most of us tend to think of as an inland county) had a longer “coastline” than the coastal county of Dorset. Due of course to using the MHW data along one side, and then the other side, of the tidal Trent. I needed to more closely define what I considered as “coastline”!

    Sometimes we need to be a little careful and first work out the definition rather than just use “some data that might be OK”.

    BTW glad you didn’t include the Isle of Scilly data for your Cornwall total since Scilly is not in Cornwall (but I’m sure you already know that 🙂 )

  2. Russell Joyce

    Gemma, are you guys in touch with Quentin Lake? He’s walking (and photographing) the entire UK coastline! Check him out on twitter.

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