The 1504 Vesconte Maggiolo Map appears to be the first map to name and accurately locate the islands of St Helena and Ascension as single isloated islands.
Ascension was shown in a similar location and named as such in the 1502 Cantino map, albeit as an archipelago. Cantino does not show St Helena.
Since the Portuguese chronicles state that Ascension was discovered by Joao da Nova in 1501 on his outward voyage to India and St Helena in 1502 on his return to Lisbon, I am curious about some earlier maps.
For example, there is the 1500 Juan la Cosa map. I have highlighted two archipelagos. These are extremely reminiscent of Ascension and St Helena but this map predates the year these islands were discovered. There are two discrepancies. First they are shown as island clusters. Second they seem to be several hundred km too far to the east.
Going further back in time, there is the “Columbus” Portolan sea chart which has been dated to the early 1490s. Once again, there seems to be an "Ascension Island" archipelago, albeit the St Helena island cluster is no longer shown.
There is a still earlier map, the 1489-90 Henricus Martellus map, that shows what appears to be an enormous archipelago that I have highlighted in the white square. If not Ascension, what is this meant to represent? Incidentally, maybe the legend next to the Ascension island cluster provides a clue but I cannot establish what it says.
It is clear to me that neither the "Ascension" nor the "St Helena" archipelagos are meant to represent the major Gulf of Guinea islands - São Tomé, Principe and Annobón – which are separately shown and reasonably accurately located on these early maps.
So, if not St Helena and Ascension, what are these island clusters meant to represent?