The Measure Of All Things is a book about the effort to measure the circumference of the earth to establish the length of the metre, about twenty years after these maps were made. The book goes into a bit of the technology used to reduce errors.

But on further reading, it looks like there was an advance in theodolite design between when these maps were published and the survey for the metre. Wikipedia provides a good rabbit hole.

https://www.goodreads.com/book/show/847635.The_Measure_of_Al...

https://en.m.wikipedia.org/wiki/Theodolite