Saturday, October 24, 2020
Which English county has the longest coastline?
A correct answer exists, which is good because this is a very ambiguous question.
The big problem is that the length of a coastline can't be precisely calculated. Whatever scale of map you choose to use, zoom in further and the length of the coastline inexorably increases. What looks like a bay may include several coves, and they in turn may have rocks jutting out into the sea with complex irregular surfaces, and just what height did you think the tide was at anyway? Indeed when mathematician Benoit Mandelbrot wanted to popularise the concept of fractals in the 1960s he chose to call his paper "How long is the coast of Britain?"
"Seacoast shapes are examples of highly involved curves with the property that - in a statistical sense - each portion can be considered a reduced-scale image of the whole. The concept of ‘length’ is usually meaningless for geographical curves."
It all comes down to how many points you choose to measure between. For example if your map of the British coastline has 2282 vertices then its length would be 3876 miles, but if you increase the number of vertices to 2,282,000 then the length shoots up to 11,023 miles. Alasdair Rae's written an excellent post explaining how the calculations work, with diminishing gifs, if this piques your interest.
But to find the longest coastline only requires putting the English counties in order, not calculating absolute values, so I only need to be consistent in my measurements. Here's what happened when I tried measuring the coastline of (ceremonial) English counties using Google Maps.
| 1 | Cornwall | 410 km |
| 2 | Devon | 340 km |
| 3 | Kent | 270 km |
| 4 | Essex | 260 km |
| 5 | Cumbria | 220 km |
By my calculations the longest coastlines belong to Cornwall and Devon, two southwestern counties with the sea on both sides. Kent is surrounded on three sides, but is a smaller county, while Essex's high showing is due to its high number of creeks and estuaries. But don't read too much into these rankings because I was only using Google Maps level 9, and zooming into level 11 changes the order somewhat.
| 1 | Cornwall | 460 km |
| 2 | Devon | 390 km |
| 3 | Essex | 320 km |
| 4 | Kent | 280 km |
| 5 | Cumbria | 230 km |
Cornwall, Devon and Essex have each put on about 50km, or 15%, because zooming in has revealed a lot more coastal contortions. Meanwhile Kent and Cumbria are only up by 10km, or 5%, because their coastlines are less indented. Bays and cliffs don't increase in length as you zoom in as much as creeks and estuaries... and this has allowed Essex to leapfrog Kent.
I could zoom in further and calculate again, but that's not a great use of my time. Instead let me switch to a potentially more accurate measurement courtesy of GB circumnavigator Quintin Lake. He's just finished five years of walking round the British coast, sticking as close to it as possible and taking some glorious photographs along the way. He's also kept very careful record of kilometres travelled and produced a summary map, which has allowed me to compile this Top 10 of County Coastlines.
| 1 | Cornwall | 724 km |
| 2 | Essex | 526 km |
| 3 | Devon | 509 km |
| 4 | Cumbria | 352 km |
| 5 | Kent | 321 km |
| 6 | Lincs | 238 km |
| 7 | Suffolk | 234 km |
| 8 | Norfolk | 232 km |
| 9 | Dorset | 222 km |
| 10 | Sussex | 219 km |
This won't be perfect either because rights of way don't always follow the coast, but because it's micro-level data it's likely much more accurate than my blunt Google Maps estimate. Notice that the distances are a lot higher than before, as you'd expect from finer detail, with Cornwall and Essex receiving a particular boost. Quintin found walking around Essex somewhat tortuous, forever following another sea wall up another inlet, which has leapfrogged this unassuming county into second place. Even better, this turns out to be correct....
Three years ago the Ordnance Survey bashed the figures by measuring the length of the High Water mark on a 1:10,000 map. They only published a top three, annoyingly, but as the ultimate arbiters of all things map-based their top three ought to be correct.
| 1 | Cornwall | 1086 km |
| 2 | Essex | 905 km |
| 3 | Devon | 819 km |
Devon is confirmed in third place thanks to its twin shores, and Essex's estuarine wiggles place it comfortably in second. But the English county with the longest coastline is definitely Cornwall, because it turns out you can answer the question even when you can't measure the data.
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