Mysterious marks and symbols hiding in plain sight

These symbols hold the key to an implausible world of 18th century military strategy, they were also a vital factor in the construction of our town and the modern world.

The initial motivation originated in 1745 when a military ordnance department decided to map Scotland in the wake of the Jacobite rising. The main problem was that the rebels had a better knowledge of the mountainous terrain than the army. What was needed was a set of accurate maps of the highlands and lochs so that the army could plan their campaigns.

The failure to adequately deal with Scottish rebellions revealed that there was a need for an accurate map of Britain. Today we take the Ordnance Survey for granted but ordnance stands for military supplies; mainly the provision of bombs and weapons. The first map was published in 1801 and England's most south-easterly county, Kent, was chosen because it was vulnerable to French invasion. A few years earlier France had declared war on Britain and Napoleon had just invaded Austria so Britain was very nervous and felt the need to build a series of defences. The OS map of Kent was a crucial factor as the early maps gave great attention to communication routes, thus highlighting the emphasis given to military use.

When you open up an OS map or use an App to organise a walk you are using information that was initially used to plan military strategies. The OS claims it is an essential service that ensures our safety and security, it delivers information to the emergency services and utilities and is crucial to the construction of our roads and railways. In the near future it promises to help build a smart transport network that will enable us to travel in autonomous cars. Indeed it feels like a friendly family organisation and yet it's origins were purely one founded on war strategy.

This Cut Datum Line Benchmark on the side of my house reveals that my house is just over 98.m above sea level; there are over 40 such marks in Leighton as it was policy to have between 30 and 40 in urban areas. These obscure marks are like secret symbols, many carved on ordinary houses. They are hidden in plain sight. The horizontal line gives the exact point at which the height has been measured. The 'broad arrow' symbol denotes that the bench mark is authorised by the crown and under the 1841 you are not legally allowed to remove one as you could get a £200 fine. The arrow symbol was printed onto prisoners' uniforms to denote that they were crown property and these uniforms were abolished in 1991.

Originally the horizontal line chiseled into the wall so that an angle iron could be placed to form a bench or leveling rod.If you have one on your house you can look it up on the OS Database ( the highest point is 132m on Bideford Green). The first datum was an arbitrary height 100 feet below a benchmark cut in the face of the tower of St. John's Church Old Haymarket Liverpool.

Years ago I worked as a chain man for a building company and we used similar surveying techniques and equipment that were used 300 years ago. My task was to set up the theodolite and and then measure triangulation distances using a metal chain and staff. The chain I carried was 22 yards long and was designed by Edmund Gunter in 1620 Ramsden's theodolite (actually a simple telescope) was invented in 1787. The Ancient Egyptians used triangulation principals to construct the pyramids and an improved method was used to create the first OS map.

Trigonometrical Pillars, such as the one on Ivinghoe Beacon, mark Longitude and Latitude whilst Benchmarks are fixed points above sea level. This was measured using a tide gauge bolt on Newlyn Pier. There are 6000 trig points and over half a million benchmarks however GPS has superseded the need for any of these.