🗺 /|\ Broad Arrows from around town. The Ordnance Survey is deeply rooted in military strategy. Mapping the Scottish Highlands following rebellion in 1745. Engineer William Roy was tasked with the initial small scale military survey of Scotland. Starting in 1747, he took 8 years to complete what was known as the Great Map at a scale of 1:36 000 (1.75 inches to a mile). Roads, hills, rivers, land cover & settlements were recorded. Roy described it as rather a ‘magnificent military sketch than a very accurate map of the country’. A surveyor called Thomas Fredrick Colby introduced height to Ordnance Survey maps in 1840 by commissioning a national geodetic levelling survey in Liverpool measured using a tide gauge. The primary levelling of the whole of Scotland was completed by 1860. This was followed by a detailed secondary levelling, to fill in a more detailed pattern of spot heights, bench marks & contours. Due to the imperfections with the levelling, it was decided to undertake a second geodetic levelling (1912 to 1921). It was at this time that mean sea level was fixed at Newlyn in Cornwall. Fixed points throughout the country were established known as fundamental bench marks (FBMs). Ordnance survey maps show a variety of detailed information OSBM (Ordnance Survey Bench Marks) being one of them. The symbol that is used on Ordnance Survey maps indicating a bench mark was approved in 1854, usually appearing as an incised horizontal bar with a broad arrow immediately below, the height of the bar accurately determined by spirit levelling. All ordnance survey maps of scale show the symbol. The name bench mark derives from the angle iron (Image 4&5) which is fitted into the horizontal cut to give a ‘bench’ or support for a levelling staff. It was policy to maintain about 5 bench marks per 1Km square in rural areas, about 30 to 40 in urban areas, & there was a policy to check & renew marks to compensate for losses due to building & road works. There used to be about half a million bench marks in Great Britain but they are no longer needed due to GPS mapping #ordnancesurvey #broadarrow #oldamersham #benchmark #crowsfoot #history #formfollowsfunction https://www.instagram.com/p/COIOQH4jXUn/?igshid=1qrin0l26bww
