On the edge of Alnwick, in Northumberland, stood Swansfield House, an elegant villa that in the late 18th century was home to Henry Collingwood Selby (1748-1839), agent to the 1st Duke and Duchess of Northumberland of Alnwick Castle. Following the lead of his monument-building patrons, he embellished his small estate with a tower, a column, and a curious gothic structure.
When the great folly builders of the 17th and 18th centuries were erecting statement buildings on the high points of their estates, they can little have known how useful they would be to the Board of Ordnance. The ‘Principal Triangulation of Britain’ was a trigonometric survey, begun in the late 18th century, which by determining precise coordinates of significant landmarks would enable highly accurate mapping. The main landmarks used were church spires, but ‘other remarkable objects’ were picked, and in the first decade of the 19th century over 50 towers, temples, obelisks, summer houses and follies made it into this category.