I'm re-reading The Dark Tower series and something is really bothering me. The gunslinger chases the man in black West across the desert until their final conflict on the coast. Then he travels North but the whole time he is travelling North, the ocean is on his right and the mountains are to his left in the East.
Did I miss something or is this just a huge geographical mistake by Stephen King?
It's not a mistake. Stephen King did this intentionally.
There are lots of references, including one in your question, in the series which tell us that places are not stable, world is moving on (literaly); because the Tower is weakening.
I have to re-read the books to find these references, but for now I've found this from Stephen King's offical website:
About the Dark Tower:
...the Dark Tower is the nexus point of the time-space continuum. It is the heart of all worlds, but it is also under threat.
Because of that threat:
In Roland’s where and when, the world has already begun to move on. Time and direction are in drift, and the fabric of reality is fraying. However, things are about to get much worse. The six invisible magnetic Beams, which maintain the alignment of time, space, size, and dimension, are weakening. Because of this, the Tower itself is foundering.
Directions are not reliable anymore. As the fabric of reality is fraying, places are not always where they are supposed to be. Things happen irrationally, unfitting the very laws of the nature.
This answer is supposition, unconfirmed, and probably better as an advisory comment.
Nevertheless, I will offer it properly as an answer.
East and West can easily be defined with reference to the apparent motion of the sun across the sky: it rises at one horizon and sets at the other.
North and South are rather more arbitrary. You can use climate or geology, but those are rather small or limited to local observation. You can use magnets — but that distinction is symmetrical. Even if you had a sacred artifact which was clearly labelled THIS SIDE SHALL BE NORTH, you could have polarity reversals if the electrons were compelled to re-orient their spins.
Fact is that distinguishing electromagnetic polarity is one of the more obviously relational properties of matter.
Indeed, due to events not well understood by contemporary scientists — i.e. they aren't even quite certain as to how the Earth's magnetic field is generated, — the Earth's magnetic field has apparently undergone something known as a “geomagnetic reversal”, where the magnetic north and south drastically and rather abruptly swap places. I.e. the polarity is reversed.
There are ways to measure such reversals, and they don't contradict what I said earlier about magnetic polarity being symmetrical and nomenclature arbitrary.
Anyways.
It is quite possible that Mr Stephen King knew about such a thing, and that he deliberately made the compass in the Gunslinger's world a chiral mirror of ours as a way of distinguishing it from ours. The Gunslinger's world, being older, has probably experienced more such geomagnetic reversals than ours, and it quite possibly is in the middle of a period where, if it were placed alongside our own, would have magnetic north where our world has magnetic south.
Ours:
N
│
W ─ O ─ E
│
S
Theirs:
S
│
W ─ O ─ E
│
N
