Calculating curve of a part of a circle

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So, a friend of mine gave me this video about why the earth is supposedly flat: https://www.youtube.com/watch?v=igoizIP7f-g

And in the first test that they conduct, the people try to measure any curve of the earth over a range of 40km.

As the circumference of the earth is 40.000km. ~1000 times the tested distance.

I was wondering if it is possible to calculate how much the curve is of a 1000th of a circle.

And is it even possible to see any curve over this small distance?