Long before the European Age of Discovery, the Polynesian peoples had explored and populated the huge expanse of the Pacific. Indonesians had crossed the Indian Ocean to populate the island of Madagascar. The Vikings crossed the Atlantic to North America. During the Middle Ages Arab traders and huge Chinese expeditions sailed the Indian Ocean but European geography remained bound by ancient theories and scriptural interpretation.
It was the Arabs who developed navigation as a mathematical science measuring the height of the sun at midday to calculate their distance from the equator (latitude). Jewish map-makers in Spain and Portugal combined European and Arab learning in the late middle ages.
When Duyfken sailed to the Indies, Mercator had devised a map on which a straight line represented a line of constant compass course, but the new Mercator's projection was not much used by sailors. Navigators could find their latitude but had no reliable way to find their longitude (distance east or west). Yet, the second Dutch fleet to sail to the Indies, on its return voyage sailed across the Indian Ocean and around the south of Africa without sighting land, then sailed up the South Atlantic to the tiny island of St Helena. How could they find their way to a small island after sailing 9,000 miles out of sight of land?
The original Duyfken voyage map
They knew that to the south of Africa was an area of relatively shallow sea (about 200 metres). They used a lead weight to sound the depth of the sea and put something sticky on the weight to bring up sand or mud from the bottom. In that area they expected to see a type of seaweed which they called "trompen" and many seabirds. These clues, plus their latitude, told them when they were south of the Cape of Good Hope so that they could change course to sail up the Atlantic. They headed for a point a little to the east of St Helena and when they reached the latitude of the island they turned due west to sail along the latitude until they found the island.