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)
In: Other
You use [land surveying](https://en.wikipedia.org/wiki/Surveying) to determine that.
You can determine your location on earth with quite a high accuracy by knowing the time observing the moon, stars, and the sun. With a simple [sextant](https://en.wikipedia.org/wiki/Sextant) and an accurate clock, a skilled navigator can determine your location anywhere of the earth with accuracy if 0.25 nautical mile (460 m, .28 miles) With telescopes on the ground you can do a lot better than if you do lot of observation.
You use triangulation to get higher accuracy and elevation. Put telescopes on towers, mountains etc an measure the angle. The distance can be over 100 km. if you just measure line some other way you can calculate everything else. That includes earth curvature as the sum of the angle in the triangle is larger than 180 degrees because they are on a sphere.
The baseline you measure is shorter and measure with a chain or metallic rods where if you have 3 end to end you can move the last and put it in front. That way you can get a baseline a few km long on a flat area of land
That is how the [Great_Trigonometrical_Survey](https://en.wikipedia.org/wiki/Great_Trigonometrical_Survey) of India was done between 1802 and 1871. You can see the [grid over India](https://upload.wikimedia.org/wikipedia/commons/0/00/1870_Index_Chart_to_GTS_India-1.jpg)[ and the observation](https://upload.wikimedia.org/wikipedia/commons/0/00/1870_Index_Chart_to_GTS_India-1.jpg)[ point.](https://upload.wikimedia.org/wikipedia/commons/0/00/1870_Index_Chart_to_GTS_India-1.jpg) It was in 1847 in this survey that they from a distance up to 240 km (150 mi) away observed a mountain peak in Nepal that they were not allowed to enter. The calculation resulted in a mountain with an elevation of 29,000 ft that was changed to 29,002 ft so it di not look like an approximate value. The mountain was named after the former leader of the survey George Everest. Tath is how the height Mount Everest was determined and how it got the name.
Later areal and satellite images have been used and today most is done with GPS receiver where the surveyor models with post-processing get a few cm accurate including elevation location anywhere on earth.
So for river length and county area you make maps of them in a way like that and the measure on the map.
Triangles!
Once you measure the distance between two points (A and B), you can choose any 3rd point. Measure the angles at A, B and C, and now you can fill in the sides BC and AC with a little trigonometry.
You can now use C as a base point for your next triangle. Maybe you figure out triangle ACD. Now you can use C and D as a base and go even further away from your initial measurement.
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.)
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