On a virtual tour of Christie’s for the auction organized by the foundation that manages the estate of the heir to oil magnate Gordon Getty and his wife Anne, Alex Clausen of Barry Lawrence Ruderman Antique Maps Inc noticed an old nautical chart known as a portolano .

Christie’s had dated the map to between 1500 and 1525 with an estimated value between 100,000 and 150,000 dollars. Getty and his wife had also bought it from Christie’s during an auction in 1993. The couple had the map restored and for years it hung in the library of their San Francisco mansion. They then paid about 56,500 British pounds for the map, about $85,000.

Clausen, president of Barry Lawrence Ruderman Antique Maps Inc., who scours auction, estate sale and other dealer websites for antique maps, manuscripts and important historical documents that have been forgotten or ignored, sometimes for decades, suspected it was much older. In October last year, he and his team bought the map from Christie’s for $239,400.

After months of research – including pigment analysis, multispectral imagingas well as consulting scholars—Clausen and his colleagues determined that the parchment map was likely created in Venice around 1360.

The new date makes the Getty Foundation’s acquisition the oldest portolan map in the US — older than those belonging to business titans and portolan hunters Henry Huntington, J. P. Morgan or in Museums and Libraries. Alex Clausen emphasized, according to a report in the Los Angeles Times, that his find is worth $7.5 million.

The map is the only complete 14th century portola known to exist outside of Europe and very important for understanding the evolution of European Mediterranean cartography, said Richard Flederer, an independent academic specializing in the field.