How do you restore land the flood erased?

That time, that place

In Egypt the Nile flooded every year. When the water fell back it left rich soil for farming — but it also washed away the stakes and ridges marking field boundaries. And taxes were assessed by area. Fail to restore the boundaries and the state's finances collapse.

Why this question

So each time the water receded, surveyors went out. The Egyptians called them 'rope-stretchers.' Eyeballing wouldn't do: to redraw boundaries without dispute, any two people measuring had to get the same answer.

What was found

Knot a rope at even intervals and you have a ruler. Make a triangle whose sides run 3, 4, 5 knots and one corner comes out exactly square. Now fields can be divided cleanly. Methods for computing area accumulated too, and the Egyptians got surprisingly close on the area of a circle. Land-measuring — that is where geometry begins.