What is a Bench Mark
Quite simply it is a permanent mark used by Ordnance Survey to establish height at that point.
There are several types in use. The following definitions have been taken from OSGB.
Some may not be used in NI.
Flush brackets consist of metal plates about 90 mm wide and 175 mm long cemented into the faces of buildings. They are fixed where possible at intervals of about 1.5km along all the lines of geodetic levelling and also at important junction points at along the secondary lines and in some tertiary areas. A Flush Bracket is also set into the side of most Triangulation Pillars. The recorded altitude refers to the small horizontal platform at the point of the broad arrow marked on the plate face. Each bracket carries a unique serial number.
Fundamental Bench Marks are marks that have been constructed at specially selected="selected" sites where foundations can be set on stable strata such as bedrock . In this way the likelihood of movement of the mark is minimised. They provide a stable network for levelling and there is approximately one every 40km. Each FBM consists of a buried chamber with a brass bolt set in the top of a granite pillar. Position and height above Ordnance Survey Newlyn Datum is known.
A Projecting Bracket is a type of bench mark. This type of bench mark was used in the early stages of the second geodetic levelling between 1912 and 1921. They are usually found on the abutments of railways and canal bridges. The reference point is the raised stud on the platform of the bracket.
A bolt bench mark is set in a horizontal surface, such as a concrete block. They are 60 mm in diameter mushroom headed brass bolts engraved with an arrow and the letters 'OSBM' - Ordnance Survey Bench Mark.
A cut bench mark is the commonest form of bench mark. It consists of a horizontal bar cut into vertical
brickwork or similar surfaces. A broad arrow is cut immediately below the centre of the horizontal bar. They are
established along all classes of levelling lines. The height value refers to the centre of the horizontal bar.
[some have a bolt instead of the horizontal bar and some have both. On this site these are all classed the same. Other sites class a cut BM with a bolt as 1GL Bolt.]
Rivet or pivot bench marks are bench marks on horizontal surfaces which have a small brass rivet inserted as the reference point. An arrow symbol is cut alongside where this is possible or they may consist of a small hollow cut for a pivot to be inserted at the reference point, these are termed pivot bench marks and also have the arrow symbol.