The British had particularly strong links with Madeira. The island had been under Portuguese rule since its discovery in 1418 - Portugal being the country with which England had (and still has) its oldest diplomatic alliance - based on the Treaty of Windsor of 1386. This alliance was further strengthened when, in 1662, Charles II married Catherine of Braganza. As a result Englishmen were allowed to settle on the island. Many of the new arrivals would later become involved in the lucrative Madeira port trade. It is likely that William Johnston, the author of the present chart, was involved in the industry, as one William Johnston is recorded as a partner in Newton, Gordon, and Johnston from 1777-1791. The company - now trading as Cossart, Gordon & Co. - was established by Francis Newton in 1745 and is the oldest company in the Madeira wine trade. These strong links were coupled with the island's strategic significance as a principal port of call for the burgeoning transatlantic trade.

To the left of the main chart is a 'Plan of the Road of Funchal' (Johnston's is the first chart to include one) with detailed soundings, anchorages, and observations made by Thomas Howe in 1762, communicated by Alexander Dalrymple to the admiralty; above is a coastal profile 'View of the City of Funchal...'.

We are aware of only one other recorded example where the charts are joined, in the British Library.