Dalrymple is the father of Admiralty Charts, and compiled more than a thousand nautical maps in his lifetime, charting many of Britain's trade routes for the first time.
He was appointed Hydrographer to the Admiralty in 1795, and developed the concept of the Admiralty Chart in 1800, making the most important contributions ever to the safe navigation of the oceans by navies and merchant shipping companies alike.
The portrait, valued at £137,500, is attributed to Edinburgh portraitist John Thomas Seton, and is dated 1765. No other likeness of Dalrymple is known in the UK.
It depicts Dalrymple at 28, in East India Company sea officer's uniform, with a globe map, chart and dividers beside him. It is thought to have been painted at the behest of Alexander's eldest brother (Sir David Dalrymple, later Lord Hailes) after the young seafarer had been away for 13 years in the East Indies.
The export bar means that a public collection in the UK will have the chance to raise money to keep the portrait in the country. Such bars are placed on items of significant cultural interest that are to be sold, on the recommendation of the Reviewing Committee on the Export of Works of Art and Objects of Cultural Interest. The scheme is administered through the government's Museums, Libraries and Archives Council (MLA).
"Although relatively unknown today, Alexander Dalrymple through his pioneering work on nautical charts, is a pivotal figure in the development of the global maritime industry as well as of the British Empire," said Lord Inglewood, Chairman of the Reviewing Committee.
Before joining the Admiralty, Dalrymple worked for over 30 years with the East India Company, compiling sailing directions and charts of the seas around China and India. In his position at the Admiralty, he was initially charged with turning an archive of sea surveys into a body of charts for use in the Napoleonic Wars.
The main administrative building for the UK Hydrographic Office in Taunton is named after Dalrymple, and his many charts were issued for use well into the 19th century – the last was only withdrawn in 1959.
The temporary export bar lasts until June 23, with the possibility of an extension to September 23 2008 if a serious intention to raise sufficient funds is expressed.
