Altimetry is the estimation of height. There are many situations in which is necessary to make such calculations. The situation I wish to discuss now arises when you are in an airplane, looking out a window, and wondering whether if there are any clues you can use to estimate your distance to the ground.
The normal cruising altitude on a typical flight of a jet aircraft might be 30,000 to 36,000 feet, that is, roughly 6 or 7 miles. At that height, the large features of a landscape can be perceived, including rivers, coastlines, mountains, lakes, boundaries of forests, large crop fields.
Although I don't have any way of measuring height, I try to imagine a somewhat object measurement of height by noticing what I can distinguish in the landscape below me. This is especially interesting if the plane begins a long slow descent, so that many things in the landscape slowly come into greater focus.
A typical divided highway, with two or three lanes in each direction, might have a width of 100 feet. At what height can you discern the highway? When can you actually see that cars and trucks are moving along it? When can you distinguish colors of the cars and trucks? When can you see that a car has a hood, windshield, and roof? When can you see the dividing lines painted on the road (6 inches wide).
A typical house might be a square 30 by 30 feet in size. As you descend, you can see the house, and if at an angle, you may see the roof separately from the house. Lower down you can see windows in the house, and then individual panes in the windows.
I will try to think of a few other things I've noticed when I have the chance.