Mapping the history of Ordnance Survey's trig points

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A handful of fans visited Northamptonshire to mark the 75th anniversary of the Ordnance Survey's first trig pillar, the triangular post used by surveyors, near Cold Ashby.

More than 11,000 were built across the country and by measuring angles and using trigonometry, surveyors could calculate distances between the pillars.

It resulted in maps accurate to just a few metres, as Joel Mapp reports.

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