The story presented in the article is actually a simplified example presented by Cleomedes to explain the method. It is unknown what Eratosthenes did to derive

The story presented in the article is actually a simplified example presented by Cleomedes to explain the method. It is unknown what Eratosthenes did to derive the calculation, but according to Eratosthenes, he used existing surveyed measurements to derive the value, likely with the understanding of the relative locations of those measurements beforehand.