>the Earth is flat and you stand at 0 N 0 E, how do you move in a straight line East of there?

This is something that was more or less solved a long time ago with surveying instruments. You don't have to move in a straight line, you build triangles out of sight lines.

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fluoridation 2 hours ago | parent [-]

I can kinda see how that would work, but it presents the challenge that whatever route you plan, it cannot go over water for more than a few kilometers.

	▲	roywiggins an hour ago \| parent [-]
		I don't think it would be that different than the arc measurements that were actually done, you triangulate a bunch of points to work out distances and angle sufficiently precisely: https://en.wikipedia.org/wiki/Arc_measurement