Taprobana is Borneo

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!

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

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.

 

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. [I tried it and it didn't work very well.]

Argument #2: 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

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

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

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 (article) 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.