system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How were cartographers able to make increasingly accurate world maps as early as in the 16th century, when we weren't even airborne?
gpt
They used *triangulation*, or measuring triangle shapes. You start by picking two places, and measuring, with a measuring chain, rod or a trundle wheel, the distance between them. Then you choose a third point that you can see from both places, and measure all the angles. Then using mathematics, you can calculate the location of that third point. Then you keep on measuring angles from your known points to find the location of further points.

Eventually you have mapped out triangles all over your country, you know the shape of the triangles and the location of every point, and you can draw an accurate map of your country.