To take just two examples, how do we know that the River Shannon is exactly 345 miles long, or that Slieve Donard in the Mournes is 2785 feet above sea level?  The answer to such questions is to be found in the story of the Ordnance Survey, an institution which began the Great Survey of Ireland 190 years ago this year, and whose work continues today throughout our island.

When Bonnie Prince Charlie’s Highland clansmen were routed by the better equipped English Army at the battle of Culloden in April 1745, the defeated survivors melted away into their trackless glens and mountains. Hunting them down was difficult in the extreme, since roads were few and far between, and no accurate maps of the country existed.  
To remedy this defect, the Army Board of Ordnance responsible for fortifications, weapons and maps, was commissioned to survey and make an accurate map of Scotland.

Ten years later, despite rough weather, endless trudging over difficult terrain, often hostile natives, and the primitive surveying instruments of the time,  the stupendous achievement was completed, and a scale map of Scotland, excluding its offshore islands, existed for the first time.

In 1791 the Duke of Richmond,  Master General of the Ordnance, conscious of British fears of attack by Napoleonic France, and of the want of accurate maps of the nation, secured approval for the foundation of ‘the Trigonometric Survey’, and instructed it to survey, measure and map the entire landscape of England and Wales.  
This huge task, employing a process called triangulation, was begun in Spring 1792, with the survey of the county of Kent.

In ‘the Primary Triangulation’ a field survey team of the Board of Ordnance first measured out a perfectly straight and level ‘baseline’ some five miles long on Hounslow Heath near London.  

Now from each end of the baseline, they used a precision optical instrument called a theodolite to measure the angles to a prominent landmark such as a church spire or hilltop many miles distant.

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