[jameshart]

The Mercator projection has the property that lines on it are of constant bearing. You don’t need a map projection to follow a line of constant bearing - you just need to head towards the point where the same star rises every night (mostly. Over a short enough number of nights, it works, anyway).

[nabla9]

Nobody could have discovered that these points are in the line of constant bearing without maps.

(hiking in straight line and jumping into boat at the point where land ends is not how people travel)