2017-06-27 
       
       Taprobana is Borneo 
       
 (IMG) Image
       
       Ptolemy's mysterious island Taprobana is Borneo and it's quite
       obvious. But since the current consensus written in stone is that
       Taprobana was Sri Lanka, I'll spell it out with pictures.
       
       (There is an in-depth essay on this by Dhani Irwanto
       (Taprobana_is_not_Sri Lanka_nor_Sumatera,_but_Kalimantan) that I
       found after drawing these maps, making this quite redundant. But
       well.)
       
       Long story short, Ptolemy drew nice maps of the known world in the
       second century CE. The maps resurfaced in 1400, but since
       cartography hadn't advanced virtually at all during the dark ages,
       there was much confusion on where and what Ptolemy's Taprobana was.
       Somewhere along the line it was agreed that Taprobana must be Sri
       Lanka / Ceylon. Wrong!
       
 (IMG) Ptolemy's map
       
       This comparison is based on a 1535 edition of Cosmographia Claudii
       Ptolomaei Alexandrini. In these comparisons Sri Lanka is first,
       Taprobana in the middle, and Borneo on right.
       
       Argument #1: Just Look at Them 
       
 (IMG) Shape comparison
       
       If you were to describe the outlines of these maps, which one would
       be "looks like a potato", and which ones "has a bay at northeast
       and southwest, about three distinct capes"? (Borneo map is rotated
       30° counterclockwise.)
       
       To make this a bit more objective, I drew a convex hull around the
       islands, extruded each edge of the polygon into a rectangle that
       covers the whole concave part (ie. the bay), and then drew these
       pieces end to end, starting from the northmost point, going
       clockwise.About the same information on the right, as a spectrum of
       'bayness'.
       
       Note that in addition of the general profile, also the roughness
       (deepness of bays) of Taprobana and Borneo correspond very well.
       There certainly are a few features that make Sri Lanka a good
       candidate, but compared to Taprobana I don't think there is much
       question of who takes the cake.    
       
 (IMG) Profiles
       
 (IMG) Profile 'spectra'
       
       You could compare coastlines or any polygons this way by
       cross-correlating the produced values. This feels so simple that
       someone probably has written a paper on it, but I couldn't find
       any.
       
 (IMG) Position in respect of the equator
       
       Ptolemy knew about latitudes. Would he have made a 7° mistake? Keep
       in mind that determining the latitude is a simple matter of
       measuring the angle between the horizon and the North Star, a task
       every ancient mariner knew at that time.
       
       Argument #3: Surroundings 
       
 (IMG) Surrounding islands
       
       Sri Lanka is basically surrounded by non-volcanic sea floor, with
       closest islands 230 km to the west and 1250 km to east. Borneo is
       surrounded by islands, as is Ptolemy's Taprobana.
       
       Argument #4: Mountains 
       
 (IMG) Mountains
       
       Taprobana distinctly has a range of mountains going up to the very
       top of the island, whereas Sri Lanka is somewhat flat with Central
       Highlands and Adam's Peak in the southern half. Also notice
       Taprobana's small spot of mountains at the southeast cape, exactly
       the same as in Borneo.
       
       Argument #5: Rivers 
       
 (IMG) Rivers
       
       All five of Taprobana's rivers can be found among the largest
       rivers of Borneo, except for the topmost one, which also can be
       found among smaller rivers. The rivers of Borneo depicted here are
       straight from Natural_Earth dataset to avoid cherry-picking.
       
       Arguments #6 to #n: 
       
       As mentioned, Dhani Irwanto has examined this connection
       extensively, and especially provides speculation on the
       corresponding geographical names.  The only one that was obvious to
       me was Baracus Fluvius corresponding with modern Barito in the
       southeast.