Maps are beasties notoriously difficult to get right. Before even thinking about the tools of measuring and recording the information to depict, we are generally talking about depicting a 3D world in a 2D format. Then, of course, our world is a spheroid, so if we're doing a world map, we need to take the contours of the planet into consideration (yes, there is a West Wing episode that discusses this and more). Yes, I'm going to blame China Mieville for making me think about all of this again since I started reading The City & The City.
I find it oddly enjoyable to go back over old maps and globe that I encounter and see how many countries don't exist anymore (ok, it makes me feel a little old too). Even more fascinating is to study maps that were created well before modern cartographic methods. Really, people made treaties based on these things? I guess, if you had nothing better...
Before we all get too smug about how sad and simple our ancestors were, I suggest you have a go at drawing a small map, to scale. Try your back yard and see how well you do. I had to do one in college as part of a Geology class, in an open field with clearly identifiable landmarks, using Brunton compasses, but only our strides to measure distances. Give is a try. Go ahead, I can wait.
(Whistles)
Now, for the moment, I'm going to ignore seafaring charts and rudders (thank you for all that info, Shogun) to keep this conversation with myself relatively simple.
Creating a map is a simple concept that gets dicier the bigger you get. If you can see all of the things on your map simultaneously in real life, then it's relatively easy to freehand the map pretty close to accurate (so long as you take a turn standing on each of the cardinal points in the real world while looking at the finished product to make sure there is no distortion over distance). It probably won't be very accurate compared to today's detailed mapping, but a damn slice better than what de Gama had to work with. The hard part comes when the mapmaker can't see from one edge of his subject to the other. Then (if you're not just going to motherfuck it) you have to rely on measurements and triangulation. Triangulation (for the uninitiated) is the process of identifying a point in space relative to three fixed points. Sounds simple enough, but guess again.
One of the essential difficulties in triangulation is the measurement of distance. We can ignore the "what is a foot?" question for the moment as long as we are not comparing two different maps. The difficulty comes in the form of topography. If I am on a level field, it is relatively easy to step off the distances. Rolling hills increase the degree of difficulty. No, the difficulty is not from getting tired. The distance we are trying to measure is the horizontal displacement (yes, we are on a curved planet. No one said this was easy, or at least I didn't). When you include vertical displacement you create error (the lines triangulating your fixed point will cross, instead of meeting prettily). This why surveyors use the system with the "dummy stick."
The other big issue I'll cover today is, "what constitutes a fixed point?" A corner of a building is useful. A tree will probably be there for a while, but where is the point that a mountain begins? Where is the edge of the forest? How many points do you need to measure the bends of a stream or river? If we're drawing a map that is divided by a mountain chain, how do we line up those two sides? How do you triangualte a coastline? It feels like we refer to traditional cartography as an "art" for a pretty good reason.
"Here there be dragons," is part of the lore of cartography, but there were plenty of unknowns and inaccuracies within a map's boundaries as well. Some of this improved as the compass came into general circulation and you could take bearings on fixed points (essentially using the angles, as well as sides of the triangle) instead of just measuring distances. However, inaccuracies persisted. This can't be surprising, if you've ever tried to get driving directions from your computer. There weren't gangs of surveyors out constantly making sure the mountains were in the right places or that the rivers bent just so. Old maps were copied and old mistakes calcified. Later mapmakers created "trap streets" and even faked towns in an effort to combat plagiarism (these were intentional mistakes by the mapmaker that would clearly mark another's work as a copy). Yup, really.
You might be asking yourself, "what the fuck does this have to do with Fantasy writing?" Well, I'll tell you. One of my favorite bits of early Fantasy reading was from the minds of Weiss and Hickman, using an old map to lead a party of adventurers to a sea that no longer existed. It's an easy way to get lost, to mistime an army's pincer movement, to unintentionally cross a boarder, and so on. Bad maps (and heroes with a bad sense of direction) are a simple plot device that can be put to any number of uses. Why don't you have some fun with your today?
West Wing clip - https://m.youtube.com/watch?v=qZNeUkwB3nM
Surveying Supplies - http://www.engineersupply.com/surveying-equipment.aspx
Map Traps - http://www.gislounge.com/map-traps-intentional-mapping-errors-combat-plagiarism/
Surveying Wiki - http://en.m.wikipedia.org/wiki/Surveying
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