Simplify where possible!
Consider making several 1-page maps, with 1 or 2 colors of explorations on each map--that way the sailing routes mostly won't intersect or overlap. Each map gets its own key. And a transcriber's note would be essential to explain the "layers".
One map could name the continents, countries, and oceans and treaty lines. If the student isn't studying lines of latitude and longitude, these can be omitted from all the maps, otherwise, just include them on this first map, which will probably become a 2-page spread to include the lat/long numbers.
The next 3 maps would omit geographic names, and just show the landmasses and oceans and exploration routes, with the routes labeled.
One could include Dutch and English.
Another could include French and Portuguese.
The third could include just the Spanish explorations.
CTEBVI Tactile Graphic Specialist