Let's say you're in the middle of a desert, with nothing but sand. Let's also assume that you have a 20/20 vision. When you're just looking there's a point that your eye can't reach name it $M$ and let your position be modeled as a point $O$. In this case what's the best approximation for the distance $OM$. Sorry If this seems unclear but I tried my best to describe this case.
Maybe this can be a better description. Let's suppose you put an object on the floor, and you started getting back until you no longer see it, then what would be the distance walked so that can happen. We will assume the Earth is perfectly spheric