Intererstin subject for a story!
Besides the technical aspects of how they measured distances and drawing techniques, the actual information and existances of maps and navigational information was a secret not to share with outsiders.
They would go to great lenght to hide and distort maps. People who knew of how they where made or simply of what they contain where important people or clever ones who sneaked there way across hidden lands or hidden vaults....
Just a few ideas of inspiration an historic knowledge
Good luck with your writing!