John Hessler's blog, where he actually discusses methods for the cartographic analysis mentioned in the article, can be found here: http://warpinghistory.blogspot.com/
Thanks for this. It's really quite astounding how good ancient cartography was, given the limitations. For all that we live in very exciting times today - and can communicate in almost real time to discuss such subjects - how thrilling, and slightly scary, it must have been to live in those times and be among the few to have a firm grasp on the fundamentals of science and the natural world.
My title was originally "Accurate map from 1275." Thankfully, some truly meddlesome mod took all meaning from the title and instead used the hackneyed print headline that was no doubt written to fill a certain space in a broadsheet.
I don't know about the rest of HN, but I had no clue what a Portolan Chart was until I read this article. My re-titling was intentional and I would imagine helped get the article to the front page.
Stop meddling or I am going to stop contributing. I understand there are times when a title is misleading or goes against guidelines, but this change is just a mod on a power trip.
The article speculates about Roman or Phoenician maps that might have been lost, but makes no mention of the more probable Arab influence. This is just my naive guess, but wasn't Arab mathematics and astronomy the state-of-the-art of the time and also probably Arabs had the most detailed information about the mediteranean region at that time.
The Arabs at the time had preserved a good portion of the scientific researches of the Ptolemaic Greeks, who were able to determine latitude (and I think longitude somewhat with many astronomical observations over time), so the relative locations of some key ports may have filtered out to some European cartographers at the time. The ability to observe latitude was possibly seeing limited re-discovery in Europe through Arab sources. For sure by the early 15th century Portuguese navigators could easily observe latitude.
But I believe these maps were created through dead reckoning. http://en.wikipedia.org/wiki/Dead_reckoning William Clark created a meticulously detailed highly accurate map of the Missouri, Snake, and Columbia rivers through dead reckoning, with the aid only of compass and sextant for latitude. (Lewis & Clark made some detailed observations for longitude, but the calculations were not performed until after the expedition was over and had nothing to do with Clark's map.)
I'll bet these maps originally had nothing to do with normal navigation (i.e. for commerce) accurate maps were for a long time military intelligence assets.
Great Britain employed a network of spies employing dead reckoning techniques to map out areas of south central Asia the British did not have access to (for military/intelligence purposes). Rudyard Kipling's novel Kim deals with this historical episode.
Not to mention all the other societies in that part of the world going back about 4000 years before that map.
It hardly seems surprising that earlier maps aren't around - by their very nature they tend to get used by people doing things, often quite risky things, so they are bound to get damaged and/or lost.
The map most discussed in the submitted article was about contemporaneous with Marco Polo, who was preceded as a traveler to China by John of Plano Carpini
was sufficiently good to serve as models for the maps mentioned in the submitted article, although the time sequence of interchange between China and the West alone does not prove any Chinese influence on the European maps.
They would have likely used a collection of East-is-Up maps to get from destination to destination. The medieval maps were maps, however they were designed to allow you to navigate from natural markings as there was no such thing as a compass, you only had the sun to tell you was going in the right direction. Similarly many of these maps were point A to point B maps, with the destination at the top, you found the path it was describing and simply followed the route.
Notable objects were used to navigate, for instance the Tree of Ténéré is still frequently drawn on maps as it was the only tree for 200km (now it's simply a statue to where the tree once was). It was a key marker for trade caravans.
They would also have likely used a great many of local navigators, as explorers still do today. Our great explorers were merely bumbling idiots who got incredibly lucky in finding amazing local explorers who had lived the land for decades.
Here's a sample map made by the mapmaker Mateo Prunes (3200x1800): http://fglorente.org/zen/albums/mapas/Mateo%20Prunes.gif