Even sailors in the days before GPS and other such modern tools had no problems navigating, basing their navigation methods on a round earth orbiting the sun.
True. And before that, other sailors based their navigation methods on a flat earth with a celestial canopy overhead.
What does that have to do with anything?
Just because one is able to navigate with a specific model in mind doesn't necessarily mean that model is correct. Thinking the world is flat will enable you to get around Chicago quite well.
Today, yes, one could easily get to Chicago. With maps, roadsigns, GPS, etc.
But crossing vast oceans or massive expanses of plains with nothing more than the sun and stars as navigation guides, based on a flat earth model, they wouldn't have worked. As mentioned before in this thread, such methods were made possible by being able to track time, based on the fact that the earth orbited the sun, the stars were in x position at x point in time, so forth and so on. As the flat earth map proposes, they never would have been able to create an accurate clock, as the suns speed in it's orbit would be constantly changing, accelerating for half the year, and decelerating the other half. This would have completely detrimented their navigation abilites, not to mention their map making abilities as well.