The simplified method works by considering two cities along the same meridian and measuring both the distance between them and the difference in angles of the shadows cast by the sun on a vertical rod (a gnomon) in each city at noon on the summer solstice. The two cities used were Alexandria and Syene (modern Aswan), and the distance between the cities was measured by professional bematists.[16] A geometric calculation reveals that the circumference of the Earth is the distance between the two cities divided by the difference in shadow angles expressed as a fraction of one turn.
https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_Ea...
They found a road that was almost due north and south (US 183 from Bassett, NE to 370 miles south in Coldwater, KS). They measured the elevation of Antares at its zenith one night, then again the next. From that they get the change in latitude. Combined with odometer data and some estimates for wiggles, and end with a result about 5% larger than the best-known solution.
Their measuring instrument was a Ryder van dubbed "The Van of Eratosthenes."
As an aside, later in the same episode is when I learned about GPS. https://youtu.be/yRY2SkMTafc?t=2968 . "This is the future of mapping."