# eli5, Before GPS how did they make perfectly round roads?

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Regents park in London has an inner circle over a hundred years old, it is a perfect circle approximately 1km in circumstance with uneven terrain and lots of trees.

I am sure many other cities have similar large perfect circular roads.

Eli5 How did they ‘draw’ the road on this scale before building it?

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Don’t exactly know how they did that particular park but surveying equipment (like theodolites) have been available for hundreds of years. Before that, you could use ropes of known lengths to measure relatively short distances precisely. What is needed are distance and angle measures.

On top of that humans have understood geometry for two thousand years (at least). And once you know geometry and trigonometry, constructing shapes using angle and distance measures aren’t too complicated.

The tool theodolite in various different versions has been around for hundreds of years and is still in use today. https://en.wikipedia.org/wiki/Theodolite

The tool theodolite in various different versions has been around for hundreds of years and is still in use today. https://en.wikipedia.org/wiki/Theodolite

Don’t exactly know how they did that particular park but surveying equipment (like theodolites) have been available for hundreds of years. Before that, you could use ropes of known lengths to measure relatively short distances precisely. What is needed are distance and angle measures.

On top of that humans have understood geometry for two thousand years (at least). And once you know geometry and trigonometry, constructing shapes using angle and distance measures aren’t too complicated.

Don’t exactly know how they did that particular park but surveying equipment (like theodolites) have been available for hundreds of years. Before that, you could use ropes of known lengths to measure relatively short distances precisely. What is needed are distance and angle measures.

On top of that humans have understood geometry for two thousand years (at least). And once you know geometry and trigonometry, constructing shapes using angle and distance measures aren’t too complicated.

The tool theodolite in various different versions has been around for hundreds of years and is still in use today. https://en.wikipedia.org/wiki/Theodolite

Probably by calculating the curvature at that size and then picking points at the correct distance where it’s possible to line yourself up directly facing the centre point, and build out from there. I’m guessing at the time the ahem trees in the area wouldn’t be particularity high and getting a vantage point to maintain accuracy wouldn’t be that difficult.