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How could the Ordnance Survey be accurate within inches?. Explain the triangulation method in plain language, emphasizing that one known side and measured angles allow a country-wide network to be computed. End with the book s striking accuracy claim that the calculated distance differed from actual measurement by less than five inches.

How the Ordnance Survey used triangulation

The book explains that the Ordnance Survey mapped Britain by building a chain of triangles: surveyors first measured one base line very accurately, then measured the angles of the connected triangles and used trigonometry to calculate the other sides[1][2].

  • The principal base line was measured on Salisbury Plain and was about five miles long[3].
  • Once that starting side was known, the calculated length of one triangle became the known side for the next triangle[4].
  • By repeating that process, the network could be extended step by step across the country[5].

The book's accuracy claim

To check the work, the surveyors measured a separate verification base at Lough Foyle, nearly eight miles long, and compared it with the distance produced by the triangulation network[6][7]. The book says the calculated distance and the actual measurement differed by less than five inches[8].


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