In perspective drawing, as a mental altitude, the viewer's eye-level is the horizon, struggling with the various forces that inform our understanding or perception of a place.

Earth's radius is between 6357 km (polar) and 6378 km (equatorial) - one meter used to be defined as one ten-millionth of the length of the meridian from pole to the equator.

So, you can calculate the distance to the horizon by multiplying 113 times the square root of the altitude (perspective drawing).

Drawing in perspective has a unique angle that sets you apart from the competition. Still, a very nice thing to write about.