Last 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?
The level of the sea rises and falls all the time due to the tide which has daily, seasonal and yearly (or longer) cycles, and also due to weather patterns. Tidal levels are measured with a Tide Gauge which is, in its simplest form, a tube lowered into the sea that has a counter balanced float inside. The float is connected to a recorder (think of something like the pen and moving graph of a seismograph for earthquake monitoring) that records the changing height of the water against a fixed ‘benchmark’ point on land. Over a long period of time (multiple years) the average of all the tidal heights gives mean sea level (MSL) relative to the benchmark.
The MSL heights of other benchmarks can be observed relative to the original next to the tide gauge. In this way a network of benchmarks can be spread across the country leaving behind a reference point to height above MSL. These benchmarks can be used as reference points in surveys to height features such as roads and buildings, relative to MSL. Heights of hills and the contours are usually surveyed from aerial photographs that are themselves referenced to MSL, by taking measurements on the photo to well defined points that have an MSL height. As mentioned in last month’s article on a growing hill the modern method of surveying an MSL height is not to make observations to benchmarks, but instead to use GPS.
On mainland Great Britain, MSL is defined at Newlyn in Cornwall where tidal observations between 1915 and 1921 were used to compute the MSL datum. This datum is often referred to as ODN, ‘Ordnance Datum Newlyn’. The larger islands, such as Isles of Scilly, Isle of Man, Outer Hebrides, Orkney and Shetland have their own MSL-based datums that are close to, but slightly different from, ODN due to them being observed at different times and for different periods.
So what might cause mountain and hill heights to shrink?
It is well known that sea level has risen and, for example, current mean sea level in Great Britain, according to Admiralty Tide Tables, is approximately 0.2m above the value adopted for ODN. In August we blogged about Australia ‘moving’ to keep its coordinate system in alignment to that used by GPS and said that something similar would eventually happen in Europe when the European and GPS coordinate systems got too far apart.
For MSL heights, the same principle applies. In theory, if the difference between MSL and ODN became too large then a new value for ODN would have to be considered. A rise in ODN would mean that all heights relative to it would go down so, for example, poor old Calf Top which only recently became a mountain would go back to being a hill. However, the current difference between MSL and ODN is nowhere near large enough to warrant revaluing ODN and, at a current rate of change of approximately 0.2m per century, it’s going to be a long time before a re-valuation will be considered. So the marginal mountains of Britain are safe for quite some time yet.