The angles he measured by degrees, of course.
[Enrico Giusti, Curator, Garden of Archimedes ] For the unit of length, he used the stadium,
but he didn't measure with a rope or something,
but he estimated the distance between Syene and Alexandria
by the time that a camel would need to go from one city to another.
Eratosthenes measure was rather accurate for the time.
Even for modern standards, it was say about 10%.
The source of error was essentially the estimation of the distance,
because the angles he could measure with a good accuracy.