I was looking at the Cantino Planisphere with its amazingly accurate outline of Africa. This seems even more amazing when you see that even parts of Europe like the British Isles and Scandinavia are far less accurate.

The map was supposedly made in Lisbon in 1502, and thus the available geographic knowledge of most of the coastline of Africa would have derived from a small number of voyages in the preceeding couple of decades. However when you look at example voyages, e.g. de Gama's first voyage to India, it doesn't seem like they tracked the coast closely enough to be able to produce maps of such accuracy.

What's the explanation for this? I guess it could be that the recent and important voyages to India had stimulated more accurate mapping than previously, with Portuguese navigators having little need to accurately navigate around Scotland or into the Baltic. However I'm still wondering how the voyages of Dias, de Gama etc. were able to provide this detail.

Edit - I just found an article by historian Patricia Seed explaining how the map making process worked for Portuguese explorers of the African coast, it's quite comprehensive and interesting. It also notes though that much knowledge of this exploration period was lost in the earthquake and tsunami that struck Portugal in 1755.