By: Blonde Two
Title image: Copyright Ordnance Survey 2020

If you’ve ever walked up a hill and wished you had checked out its steepness first, you’ll know how useful contour lines are on a map. These brown (on an Ordnance Survey map) lines join up points that are the same height above sea level. By counting contour lines or noticing how close they are together, you can estimate or work out exactly how much height gain (and time) you are going to experience on your route.

All very useful (thanks Ordnance Survey) but one morning outing last week had me wondering where exactly sea level (for the purposes of mapping) is. I had walked down to the seafront, had a sea swim, then walked back up the hill to my house. Presumably, as well as the 130 metres climb, I had gained a couple of metres on the map height by swimming, not only at low tide, but also underwater.

However, I didn’t know where sea level was set. I know a bit about tide gauges because my Dad wrote the software, and helped develop the hardware for many of the world’s tide gauges (including some here in the south west of England). He tells some great stories about exotic destination visits for installation and maintenance but mapping sea level for the UK wasn’t set on measurements from St Helena or Tristan da Cuhna.

Mean sea level

I recently found out that mean sea level (not mean seal level, as I just typed) for mainland Great Britain is taken from six years of recordings from a tide gauge set up in Newlyn in 1915 (before my Dad’s time). Ordnance Survey’s height calculations rely on this calculation. Interestingly some of our larger islands, including two favourite Blonde locations, the Isles of Scilly and the Isle of Man, have their own mean sea level.

Sea level rises UK

Of course we are all aware that sea levels (and presumably seal levels) are rising across the world. In the UK this is happening more quickly in some areas than others. This is because we have to add vertical land movements (north up, south down) to the predicted sea level rises, giving us as much as an 80cm rise in some areas by 2100).

As this happens it could quite rightly be argued that our mean sea level for mapping purposes should eventually rise. This would have interesting consequences when it came to the ‘mountain or not mountain‘ question. It would also mean that I would feel slightly less virtuous about walking down to the beach in the morning!


Exmoor – The Perfect Lesson in Contour Lines

Sea swimming for the first time? We have a few outdoor swimming safety tips.