Flush Bracket

detachable bracket

levelling staff on flush bracket

A Flush Brackets (FB) is a metal plate with a serial number, a reference point for levelling, and attachment holes for surveying equipment. The bracket is used for levelling; it is of no relevance to triangulation. The plate bears the letters OSBM denoting 'Ordnance Survey Bench Mark'. Flush Brackets were first used in the 2nd Geodetic Levelling (2GL) starting around 1912 and subsequently for the 3rd Geodetic Levelling (3GL). They were used on triangulation pillars starting around 1935/36. They are also included on Gravity Stations. Not all pillars were constructed with FBs; a list of those without is available. The published height of the station refers to the tip of the arrow on the flush bracket ^[1] The following details apply to FBs on pillars. The numbering on non-pillars (NPFBs) is more complex; the Bench Mark Database lists all known instances.

The 'S' prefix appears with most 4-digit brackets and denotes Secondary. The letter was omitted, due to lack of space, for 5-digit numbers. Exceptions to this occur with brackets numbered between 2900 and 2999 where the 'S' is omitted, and for the 'BsM' series where it is repositioned.

The _29** series flush brackets which were used on triangulation pillars in 1936 were unused leftovers from the three thousand plates manufactured for the Second Geodetic Levelling in England and Wales which had been completed in 1921.

By the time trig pillar construction started in 1935/36 the first (nearly) fifteen hundred newer S-series flush brackets had already been used on Secondary levelling lines. This is why there are no trig pillars with no very low S-series numbers.

Almost all of first of the pillar stations built were primary stations and primary substitute stations. The old 2GL _29** series flush brackets were used on these pillars concurrently with the newer S15** series FBs in 1936. This is why _29** numbers and the newer S-series up to FB No. S1624 are only found on the earliest primary stations and their substitutes where these exist.

By 1940, with pillar building continuing rapidly, and by then only using S-series brackets, the OS had reached the S2800s and halted production there for a while. With apparent forward thinking at that time, the OS skipped manufacturing any S2900 series FBs and resumed production with S3000. This means that with or without the 'S' prefix letter, the actual FB numbers used on the pillar stations were never duplicated, except in error. ^[2]. A list of Duplicate FBs is available.

The location of two FBs in the _29 series, 2960 and 2983, is unknown. It is possible that one of these was used on the original trig pillar at The Edge in the Derbyshire Peak district.

BsM is the name given to a series of Flush Brackets, numbered from S3200 to S3677 with some outliers in the S3680s, where the 'chunky' font of the lettering was enlarged such that there was no room for the S prefix before the numbers. Instead it was positioned between the 'B' and 'M'.

Where the OS constructed Hotine Pillars for Project Emily, the FB number was often (though not always) ground off. It is thought that this was to indicate that the pillar was not part of the OSGB36 network.

Occasionally, a flush bracket from a destroyed trig was reused. For example, S1817 was originally on a pillar at Otley Chevin in West Yorkshire then moved to Culter Fell in South Lanarkshire.