ELI5: How were historical maps drawn so accurately before satellites and airplanes?

Historical mapmakers drew accurate maps by walking the land, using clever math tricks, and piecing together thousands of small measurements like a giant jigsaw puzzle.

Imagine you want to draw a map of your living room, but you cannot look down from the ceiling. What would you do? You would grab a tape measure, measure the distance between the couch and the TV, and count how many steps it takes to reach the front door. Early mapmakers did the exact same thing with the whole world.

Here are the everyday tricks they used:

• Pacing and measuring wheels: Explorers counted their footsteps or rolled a special wheel that clicked every time it turned to measure distances down roads and across fields.
• A math trick called triangulation: Imagine standing in your backyard. You look at a tall tree, a garden shed, and your back porch. If you know the exact distance between the tree and the shed, and you measure the angles pointing toward the porch, simple geometry tells you exactly how far away the porch is without you ever having to walk there. By connecting thousands of these imaginary triangles from mountaintop to church tower, mapmakers covered entire countries.
• Looking at the sky: Just like using streetlights to find your way home in the dark, sailors used the sun and the North Star. By measuring how high a star sat in the night sky, they could figure out their latitude, which told them how far north or south they were.
• Super-accurate clocks: To find their position east or west, called longitude, sailors needed to know the exact time. Once inventors built ultra-tough clocks that kept ticking smoothly on bumpy ocean waves, sailors could compare their clock to the midday sun and know their location on the sea.