|Ordnance Survey benchmark, Newlyn|
Newlyn was chosen because of the stability of the site - a solid Cornish granite pier set on solid Cornish granite bedrock. However solid the foundations though, even this site might be subject to horizontal and/or vertical movement of the Earth's crust below. Careful levelling over the decades since the tidal gauge was set up in 1915, confirmed by CGPS measurements in recent years show that isn't the case.
|CGPS plot for Newlyn Observatory 1999-2010 Source:Sonel|
The CGPS (Continuous GPS) plot shows -0.03 mm/year between 1999 and 2010 (in red, to the left of the plot); it's sinking at a rate of just 3 mm per century. That's stable by any standards. Why am I telling you all this? Because of the stability of the site; the tide gauge is set in a vertical shaft in the pier with a horizontal channel to admit sea water. The record runs from 1915 and so covers most of the 20th century. Because of the stability of the gauge the tidal data can be relied on for accuracy without any adjustment for vertical movement.
|Data Source: PSMSL|
|Annual rate from 1915 to year on lower axis.|
I'll be creating a sea-level reference page for Europe very soon; I already have several plots for the UK, and half-a-dozen for northern Europe.