Bench marks

Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.

Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.

Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN.

On some of the offshore islands there are ‘Local’ mean sea level datums. These have no link in to ODN and must be treated the same way as the mainland levelling.
Global Positioning System (GPS) and the OSGM02TM (the height transformation between the European Terrestrial Reference System 1989 and the national height datums) is the preferred method of heighting used by Ordnance Survey. This method achieves the most accurate and uniform method of heighting. See www.ordnancesurvey.co.uk/gps for more information.

There are approximately 500 000 'lower order' BMs still remaining. This number is reducing due to property development, road widening and so on. The BM heights shown have not been maintained for 30 years and should not be relied upon to accurately define ODN.

There are approximately 190 FBMs, which as the name suggests are our high-accuracy BMs. These are still maintained and are still used by Ordnance Survey. They form our primary height network and, as such, are our link to the Ordnance Datum at Newlyn. They are used in the creation of the orthometric to GPS height correction model (OSTN02™). The FBMs are not available through this website. Access to FBM values will be considered on a case by case basis on application to Ordnance Survey.

Km square

All BMs are quoted to four decimal places of height in metres. Third-order BMs are actually precise to two decimal places and second-order BMs are precise to three decimal places.

Mark type

This shows the different types of BM that have been used by Ordnance Survey. The most common is the cut mark, which is, as the name suggests, cut into a vertical wall, post, standing stone and so on. Others are bolts, rivets, pivots and flush brackets, which are built into most triangulation pillars.

Description

The description is the text describing the location of each mark.

Height

This is the adjusted height of the BM in relation to Ordnance Datum.

Order

This shows the precision or class of the levelling. One or first being the highest level and three or third being the lowest.

Datum

This shows the datum point that the levelling is referred to. For example, Newlyn is the main datum we have used; Liverpool was used prior to changing to Newlyn. Most of the larger islands that have levels on them have them referred to a local datum; for example, the Isles of Scilly are based on the Hugh Town Datum.

Verified year

This shows the year when the mark was last checked visually.

Levelling date

This shows the date when the mark was last levelled and the heig