How much of the map is left for the Adventurers to explore?
First, create the source of truth for the Guide.
Next create a map for Adventurers with a starting subset of this information.
Maps present a constant danger of sharing too much information to the Adventurers.
Before aerial and orbital surveys exist, creating high quality maps required significant effort and expense.
That effort and expense make maps a rare and valuable treasure rather than a common scroll.
Maps given to Adventurers should be sparse, incomplete, accidentally misleading, and occasionally inaccurate to present more challenge and puzzle and escalate the reward gained for successfully using the map.