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.
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).
- Cornwall: 1086 km
- Essex: 905 km
- Devon: 819 km
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.