Monday, October 4, 2010

Ptolemy's Map Decoded

(Ptolemy's 150 CE World Map (redrawn in the 15th century).
German researchers have finally unraveled a solution regarding an ancient world map's distorted depiction of Germania in the second century CE giving historians, archaeologists, and other researchers a better understanding the geography of the region two millennium ago.

The ancient map in question was created in the second century CE by Egyptian astronomer and mathematician Ptolemy who had set out to create a map of the known world.  It seems surprising that someone like Ptolemy, living along the Nile as he did, would have had much detailed knowledge of the world beyond the borders of the Roman empire, especially of areas as far away as India and China.  But it is likely that he took advantage of the vast network of Roman traders, seafarers, and maps used by Roman legions.  Indeed, Rome had contact with Chinese emissaries as early as the second century BCE, so elements of the far east, though vaguely described in his work, would not have been totally unfamiliar.  As such, Ptolemy was able to create 26 color maps of the known world, drawn on animal skins, that would come to be contained in his famous treatise on cartography, Geographia.  

Geographia is composed of  two parts: Book 1, is written about the information and techniques used to compose the maps contained in the second part, as Books 2–5 are an atlas. The original work included maps, but few copies of these maps were made due to the difficulties involved in copying them by hand.  As a result they fell out of transition over the ages, and by the 1300s, when Ptolemy's work was re-discovered in the early years of Europe's renaissance, the maps were all but absent.  However, while authentic maps have never been found, Ptolemy provided the first use of longitudinal and latitudinal lines to reference most of the thousands of parts of the world that Geographia described, which allowed cartographers to reconstruct Ptolemy's world view when the manuscript was re-discovered.

Just like today, Ptolemy used the equator as the baseline for his lines of latitude, however, rather than degrees of arc, which is what modern systems, Ptolemy used length of the longest day to determine latitude.  From north to south, his map covered roughly 81 degrees of latitude, covering an area from the arctic into Africa.  From west to east he mapped from the Canary Islands in the Atlantic to China, spanning 180 degrees of the earth.  Ptolemy was quite aware that he was only mapping about than a quarter of the earth.

However, Ptolemy's system of projecting a round world onto his flat maps was not perfect, and as would be an issue with later cartographers this posed a serious problem in depicting the northern latitudes accurately.   Ptolemy mis-estimated the exact shape of the earth, believing that northern areas were far narrower and elongated than they really are.  Problems related to scale found their way into his maps as he tried to squeeze and shift lands together so they'd fit onto his projection.  Adding to the issues with his world map, Ptolemy had difficulty tying together points and places accurately as distances were misrepresenting in his attempt to connect his maps allowing more errors to slip in.  As a result, lands were distorted; the Jutland peninsula of Denmark is bent, portions of northern Germany were shunted off too far to the east, and the resulting map of the north of Europe was an unsurprisingly confusing mess.

Germania as depicted by Ptolemy in the 2nd century
This brings us back to the German researchers mentioned at the beginning of this writing.  For six years they worked on ways to fix the errors using "geodetic deformation analysis," comparing current geography and historically known archaeological sites against Ptolemy's work.  But it was hard to gauge how accurate their reference material was.  Initially, the oldest copy of Geographia was a copy produced in 1300 that is held by the Vatican.  This copy, however, was based on an an incomplete version that had been discovered by a Byzantine monk. Luckily, an even older and more complete copy was recently discovered in Turkey, giving the researches valuable clues that allowed them to finally overcome the obstacles that had been put in there way.

What their new interpretation of Ptolemy's map has shown is that many areas of Germany have had perpetual settlements on them for far longer than once thought. German towns like Salzkotten or Lalendorf, for example, have existed for over 2000 years.  Other towns indicated in Ptolemy's work that have been tied to modern locales are Treva which resided in the past of Hamburg, and Aregelia, which is now known as Leipzig.  To archaeologist, this new interpretation has literally created a treasure map for them to use, aiding in the identification of archaeological sites, and giving a better understanding of Germania's military and trade relationships with Rome and other neighboring states in the second century.

Berlin Researchers Crack the Ptolemy Code
Ptolemy's Geography and Maps
Ptolemy's World Map
Romana-Chinese Relations

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