mean sea level


25 years since the last OS benchmark

You may know about our trig pillars, but did you know that there are more nostalgic reminders of how we used to map Great Britain? Have you ever seen one of these while you’ve been out and about? If so, it is highly likely you have spotted one of our renowned benchmarks. 2018 marks 25 years since the last traditionally-cut arrow style benchmark was carved on a milestone located outside The Fountain pub in Loughton.

Read More


Are Britain’s mountains shrinking?

3DLast month we had a story about hills growing into mountains and now we blog about the opposite situation…Recent press stories about what *might* happen if rising sea levels lead to a change in the datum value used for mean sea level on OS maps, has seen some people thinking that the heights of hills and mountains might be about to shrink. They are not!

We have to measure height in Britain against a commonly agreed datum level and ‘mean sea level’ is a common value chosen in many countries. It becomes the ‘zero height’ which all other heights are measured from. As land-based creatures, it’s natural for us to think of the sea as being zero height and anything above it as being ‘high’, and consequently anything below it as having a ‘depth’. So, it’s very common to see heights of hills and mountains quoted as being ‘above mean sea level’.

What exactly is mean sea level?

Read More