system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are big scale things like rivers and mountains measured?

e.g. 

"Mt. Everest is 8848 metres tall from sea level" or "Amazon River is 6575 km long." or even stuff like "the area of the US is 9834000 sq.km."

How do they collect the stats of how tall a mountain is or how long a river is, or how much area does such a large area have. When they are all so large, they definitely won't use a ruler to do it, right? (Or am I wrong and they actually do lol)
gpt
Triangulation.

If you have two points of a known height and distance between them, you can place an observer at one point, have them measure the angle and distance from the second point to the third point, and mathematically derive the height and distance of that third point.

So let's say you take a particular measurement, like the top of the Washington Monument. You do the annoying work of measuring its height exactly (with a very long measuring tape if you have to!). Then you do the same with another point that's visible from the first - like the top of the Capitol Building's dome. Now if someone is standing on top of the Capitol dome, they can measure the height and distance of every visible building in Washington, knowing the measurer's own position and that of the tip of the monument.

Then they just keep doing that, moving on to the next tall visible point within line-of-sight. Then when they get somewhere flat, they put a long stick at a particular spot in the distance, and keep moving that way. (When you see construction crews today with looking-glasses on a tripod, and another dude holding a long high-viz yellow stick somewhere in the distance, that's what they are doing - measuring the height and distance of the ground very exactly.)

And yeah, doing that enough to cover the entire country, or up to the points of the Everest and so on, is a very tedious job. It took super-organized governments to do it.

(These days of course we can check it with GPS, too. And for something like Mount Everest, you can use a very sensitive barometer to measure the air pressure at the top and at sea level to calculate the height. Etc.)