Regarding the sources of error in ancient measurements and the use of sextants and gnomons, in particular with regard to Eratosthenes and the measurement of the Earth’s circumference: You will find all of these covered in reasonable detail in undergraduate astronomy textbooks.
Two pretty interesting sources that are available online are:
THE OBLIQUITY OF THE ECLIPTIC, by George F. Dodwell
Excerpts in this video are from chapters 2 & 5.
A History of Measures: The Circumference of the Earth
by Livio C. Stecchini
Link to Flat Earth Consequences video: https://youtu.be/EYWW8LTdNso