I've been reading a bit about ancient to medieval maps lately. And they're very interesting, and some are surprisingly accurate (Anglo-Saxon mappa mundi, especially it's outline of Scotland) but I've struggled to find any information on how they were made.

How was the information gathered, accurate portraits of coastlines, and no accurate method of time keeping. I'm just wondering what information is available on how these ancient cartographers carried out such accurate work, some even have early attempts at lat and long that is at least to consistent scale across the map. I'd love to find some more practical information for how these maps were made.