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The manuscript map, meticulously coloured and remarkably well preserved on a 2ft 1in x 2ft 7in (68 x 80cm) sheet of vellum, was drawn by London "plattmaker" (cartographer) John Thornton in 1699 and depicts North West America and Canada from Hudson's Straights south through Labrador and Newfoundland to New England and New York.

Thornton (1641-1708), a prolific member of the Thames School of map and chart makers, was a hydrographer for the English East India Company and the Hudson's Bay Company.

Rose Sanguinetti, the books and manuscripts specialist at Lawrences Auctioneers in Crewkerne, has compared this historically significant item with another map of Hudson's Bay by Thornton in the British Library. That map, dated 1685, pre-dates this recent discovery by 14 years but is undoubtedly by the same hand.

Map specialists at The British Library have suggested that the map might have been a special commission for a patron on account of the considerable detail given to small settlements on the Newfoundland coast, implying an interest in the local fishing enterprises.

Its surprising appearance in The House of Glennie, by Huntly, Aberdeenshire, Scotland is explained by the business interests of the late vendor's father, Harold Fortington, who had links with Canada and North America before the Second World War. When Fortington's daughter, Mrs N.A.J. Moulton-Barrett, died in 2009, the map was found, on a dusty attic shelf by a water tank, by Lawrences' director Anthony Kilroy during a probate valuation.

It is expected to realise £50,000-80,000 when it appears at Lawrences' auction on January 17.

By Roland Arkell