system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is the height of very high mountains measured accurately?
gpt
Today they just go up there with a GPS receiver or scan the earth with a satellite based radar.

Prior to those technologies you could triangulate the height. Create a triangle using the peak of the mountain as one point, and two fixed and known points on the ground as the other two.

So long as you have the distance between the two known points, and can measure the angles it's simple trigonometry.