Ordnance Survey's
Levelling
of Great Britain

Frequently Asked Questions

What is Levelling anyway?

Surveying and mapping by the OS has a basic introduction to levelling

When was the Levelling carried out?

It was first started as a national project in 1840 and continued until 1980's, where the levelling network as basically abandoned in favour of GPS methods. See the Timeline

What is a Benchmark?

Benchmarks are various marks made by the Ordnance Survey to record height. If the exact height of one Benchmark is known then the exact height of the next can be found by measureing the difference in heights, threw a process of sprit levelling.

What Type of Marks Are used?

Cut Benchmarks	BM	These where cut into rock or stone to create a horizontal line with an arrow point to it. (Example)
Rivot		Similar to the Cut Benchmarks but with a small brass rivet inserted as the reference point. (sometimes they still have the horizontal cut)
Pivot		Similar to the Cut Benchmarks but with a small hollow cut for a pivot to be inserted at reference point. (sometimes they still have the horizontal cut)
Bolt	Bolt	These are small concrete blocks buried near the surface. The top serface is 30cm square, in the center is a Metal Stud bareing 'OSBM' and the arrow mark.
Projecting Benchmarks	PB	These are brackets mounted on walls that have a small pertursion with a Stud on thats the actual BM. These where believed to be discontinued due to them being damaged.
Flash Brackets	FB	These are numbered brackets that are mounted on walls and on the side of Trig Points to record height. Being numbered is what has got many into the study of the Levelling network and being as a vast number are on Trig Pillars into bagging trigpoints (T:UK)
Fundamental Benchmarks	FBM	These appeared in the 2nd Levelling and used and upgraded in the 3rd. They are usually granite pillars with underground chambers, they are sited directly on bedrock, so that they don't move over time as a other types of Benchmarks could. The heights calculated during 2GL where reused unchanged for 3GL.
The Fundamental Benchmark		This is the BM that all others measure their height from. 15.588 ft above Sea Level. More...
Surface Blocks/ Rivets/ Bolts etc		These are generally used as part of the newer GPS Passive Network. See T:UK Types for a good overview.

What Type of Flash Brackets are there?

(this is my understanding of the various numbering used)

Example	Known as	Description
0234	Un-Prefixed	These first appeared in the Second Geodetic Levelling to create more prominent marks than Cut BM's and also more robust than the Projecting Brackets. Some are also used on Trig Pillars (mainly primaries).
_0234	Un-Prefixed
S0234	Secondary or S Series	These where used on secondary lines and also the majority of trigs use S series. The S series seem to be unique in using 0 prefixes for low numbers to make up to 4 digits.
10234		Once 9999 was reached the S prefix was dropped but they are still part of the S series. It's possible that some Tertiary lines used these Plates, probably at strategic junctions.
0234/S		Believed to be part of the S series just with the S under the main digits. (probably the early S-Series, but the significence is unknown)
S\0234
S/0234		Believed to be part of the S series just with the S above the main digits, between the B and M. (sometimes known as BSM) - the significence is unknown
G234	Geodetic or G Series	These started in a Scottish 2GL Lines but mainly where used during the 3rd GL. They never appear on trigs.
G/345	Geodetic or G Series	Probably part of the G series just with the G above the main digits, between the B and M. (known as BGM, possibly) - the significence is unknown
L4	L Series	It seems thats theres a L4 FB on Tower Hill. Quote from "The Levelling Handbook - 2nd Edition 1972". 'In 1932 16 Fl Brs were emplaced in the London area, bearing the numbers L1 to L16' Tentative locations have been found for some.