Good question. I found a reasonable answer . It includes .
Eratosthenes and the Mystery of the Stades - Distance from Alexandria to Syene "That the distance between Alexandria and Syene is 5000 stades.
There is little doubt that Eratosthenes got this figure directly from his earlier map of the known world [4, p.62]. How he initially obtained this value is a controversial question which may never be answered, but it is doubtful that Eratosthenes actually measured the distance himself [5, p.154 ]. The writings of Strabo the Geographer (ca. 20 BCE) suggest that the lands along the Nile were measured every year."
There is little doubt that Eratosthenes got this figure directly from his earlier map of the known world [4, p.62]. How he initially obtained this value is a controversial question which may never be answered, but it is doubtful that Eratosthenes actually measured the distance himself [5, p.154 ]. The writings of Strabo the Geographer (ca. 20 BCE) suggest that the lands along the Nile were measured every year."
