Ordnance Survey Bench Marks in Henley
Bench marks (BMs) are survey marks made by the Ordnance Survey to record height above their standard reference point of sea level at Newlyn in Cornwall. If the exact height of one BM is known then the exact height of a neighbour can be found by measuring the difference in height through a process of spirit levelling. A series of linked BMs is referred to as a line.
Most commonly, BMs are found on buildings such as churches, schools, and pubs or other semi-permanent features such as milestones and bridges. They take various forms including cut marks (a horizontal cut at the levelled height, with a carved arrow below), rivets on horizontal surfaces (many on culverts and now buried), and metal plates known as flush brackets (which are also found on the more familiar trig points on hills).
The first survey of England and Wales (called 1-GL) was carried out during 1840-1859. The line from Basingstoke to Coventry which passed through Henley was levelled between 6th November 1850 and 7th June 1856. (description and heights of the marks in the Henley area and their grid references derived from studies of old maps). Among the sites used in Henley several survive, the best example being on St Mary's Church (full description). This is a 'bolt' which is a cut mark variant which has a bolt at the point where the datum and arrow cuts meet). There was also a cut mark on the original market cross (the obelisk now moved to Mill Meadows), and there are probably the remnants of a cut mark on the Packhorse Inn (now 19 Northfield End).
There are at least 100 BMs within 5km of the centre of Henley, mostly cut marks dating from the 1960s and 1970s. Many are on private houses, and some on business premises such as OPAL Picture Framing, Lloyds TSB Bank, Saracen's Head pub, and CMP Motor Parts. There are two flush brackets (each of which carries a unique serial number) in Henley: these are No. 1026 on Henley Bridge and No. 1025 on Denmark House.
There are still a few BMs to be 'discovered' around Henley, for example a rivet on the concrete base of the railings at the entrance to South Lodge, Fawley Court. There may be other marks that are not in Ordnance Survey records either because they have been abandoned in favour more modern replacements or the structure on which they were cut is no longer stable enough to provide a reliable height measurement (e.g. milestones that have been repositioned).
Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion. Most surveying is now achieved with satellite techniques although there are still 190 ground truth stations known fundamental bench marks (FBMs). The FBM nearest to Henley is at Wallingford.
From these FBMs tens of thousands of lower-order BMs were established. There are probably about 500,000 BMs still in existence in the UK, so plenty still to find for those sad people who have a hobby of searching them out.
Article kindly provided by Roger Templeman. Please email with questions.