Nautical Maps, Criss Crossing Navigational Lines. Why so many starting points?

[Azélor]

I don't know a lot on the topic but the position of the magnetic pole have always been roughly located in the same region for the last couple centuries. Moving slowly so that people could not notice the changes until recently due to some technology, I presume.

A polar inversion is possible, where the north and south switches position over a certain period of time. If that happens, the compass will point to the south pole.
A weakening of the magnetic field is also possible. In that case, several pairs of poles could form. The compass will indicate the closest ''north'' poles, and it can be located anywhere on the planet.

I don't know how fast the changes can occur and nobody would try to create a navigation system on something that random and slow.

Sailors can also relies on the position of the stars o indicate their position using a sextant for example.

[Tonnichiwa]

Yes, the sextant is one tool, but it is only useful if you can see the stars on a clear night. When the clouds are out, you are out of luck.

One of the reasons I mention that magnetic north was one of the variables is because it does change quite a bit in the Bermuda Triangle and historically there has been a lot of naval traffic through the Bermuda triangle. I forget the name of some of the other ones but there are a few other places on the earth where magnetic north changes drastically too.

[ladiestorm]

when you were talking about using the protractor, snapping it twice, then drawing your line... that reminds me of what they do in carpentry... using a chalk line, marking the points on where it needs to be cut, then using a chalk string to line them up to make sure the line is straight, then marking the line to follow. Am I close?

And I sucked at trig as well... I was best at geometry, for some reason.

[Hai-Etlik]

so, I am assuming that a compass would always point to magnetic north. Correct? But you just said that Magnetic North moves. So how would a compass truly be useful, then? Magnetic North may change, but places on a map generally don't. So having a compass that points to a point that changes... wouldn't that throw sailors off course? Or is that another reason for the different rhumb lines? Compensation for the changing of Magnetic North?

A compass points along the magnetic field. Besides the magnetic poles not lining up with the geographic poles, the magnetic field can have distortions in it so that along the field isn't necessarily straight to the magnetic pole

At any particular point on Earth (at least at those far enough from the poles that magnetic compasses work at all) there's an angle between the lines of magnetic force, and the meridians of longitude. This is called the "declination". And yes, it varies over time as the magnetic field shifts. The magnetic poles are also not antipodal (A line between them through the earth doesn't go through the exact centre). The exact definition of the magnetic poles are the points where if a compass needle were allowed to rotate freely in 3D, it would point straight up or down.

There are also geomagnetic poles which are distinct from the geographic and magnetic poles. It's also worth noting that magnetic polarity is defined in terms of the magnetic pole of the Earth that attracts them, and opposite polarities attract one another, so the magnetic north pole has a magnetic polarity of south. (and vis versa, the south magnetic pole is a north pole)

Rhumb lines are essentially an alternate way to represent the same information as a compass rose: This map preserves bearings (Not all maps do, and you should never place a compass rose or rhumb lines on a map that doesn't.), This map is oriented this way (This can also be indicated with a "north arrow" without saying that other bearings are preserved), here are other measured bearings (Essentially it acts as a built in, already aligned protractor).

On a modern chart, rhumb lines would be true/geographic bearings, however, on a modern char they would also be an affectation as they aren't used seriously any more. Real modern navigation charts use compass roses that look like overgrown protractors, sometimes doubled up with both true and magnetic bearings (Other things changing will render a navigational chart out of date long before a change in declination does). On historical portolan charts, accuracy as low enough that they wouldn't necessarily have made the distinction or would have been happy to orient the map magnetically. Measuring declination wouldn't have been beyond them at the time though as true north can be found by looking at the stars. It's not practical to represent both magnetic and true bearings at the same time using rhumb lines. There would just be to many lines. On a map that is not a navigational chart, rhumb lines, and for the most part compass roses, are also affectations.

The main benefit of rhumb lines over roses is that they make bearing information available all over the map. On the down side they are less precise (you cant represent as many bearings), they can't include both true and magnetic at the same time easily, and they add a lot more clutter (One of the most important rules of cartography is to not include anything in the map unless you really, really need it to accomplish the goal of the map). With roses at strategic points over a chart and a pair of parallel rulers, you can transfer bearing measurements to or from any rose more precisely and almost as easily as with rhumb lines.

So, rhumb lines are only a normal part of a very specific kind of map which is a result of navigation reaching the point of needing actual charts with precise bearing information, but not so precise that the downsides of rhumb lines kill them off again. Outside of such charts, they are just being copied because they look cool, even though that's not really a good idea. Real cartographers certainly did this, and it's not out of the question to try to replicate it in fantasy maps, but it is important to understand it before doing it yourself. Considering the technology available and the purpose that the notional "in character" cartographer has in making the map is also important, not just for this but for making many other decisions if you want your map to feel real.

One thing you should try to avoid is fearing blank space. Don't toss in lines and noise and textures to fill up blank spaces. The blank spaces are what give the features of your map that are important shape and definition. Filling up the empty areas with meaningless noise takes away from the visual importance of the things you are trying to present. Everything you add to a map takes away from all that you had already added. Sometimes it adds more than it takes away, but it always takes away something.

[ladiestorm]

ahh, the Devil's Triangle... I've always wondered... what is it about the Bermuda Triangle that causes so many problems? I have to confess, the Bermuda Triangle has ALWAYS fascinated me. One of the few of Earth's mysteries that has never been really explained.

[Tonnichiwa]

when you were talking about using the protractor, snapping it twice, then drawing your line... that reminds me of what they do in carpentry... using a chalk line, marking the points on where it needs to be cut, then using a chalk string to line them up to make sure the line is straight, then marking the line to follow. Am I close?

And I sucked at trig as well... I was best at geometry, for some reason.

Sorry, I meant parallel ruler, not protractor. No, not really. When I say snapping it twice, that means you are using a parallel ruler that extends and snaps back together again. To snap it means to bring them back so both sides touch again, instead of being extended. If you have to extend them out more than twice, your measurement is usually going to be way off, so they try not to do that.

[Ilanthar]

By Ladiestorm
I have to confess, the Bermuda Triangle has ALWAYS fascinated me. One of the few of Earth's mysteries that has never been really explained.

Actually, there are some interesting hypothesis. One of the most credible that I like is the release of a big bubble of gas from the ocean floor. There are some of those gases (similar to clathrate if I recall well) that could dissolved in the water but lowering the density of it. Consequently, when the "water-gas bubble" is close to the surface, a ship wouldn't be able to float anymore and "fall" until it is in "just" in sea water again, but too late...

[Falconius]

Actually, there are some interesting hypothesis. One of the most credible that I like is the release of a big bubble of gas from the ocean floor. There are some of those gases (similar to clathrate if I recall well) that could dissolved in the water but lowering the density of it. Consequently, when the "water-gas bubble" is close to the surface, a ship wouldn't be able to float anymore and "fall" until it is in "just" in sea water again, but too late...

That is a very cool theory. Except it doesn't explain all the missing planes.

[waldronate]

That is a very cool theory. Except it doesn't explain all the missing planes.

The argument claims that the methane bubbles (which will rise in the atmosphere) change the fuel-air mixture enough to cause the engines to misfire, lose power, and the pane to stall and ditch in the ocean.

The minor detail that pretty much every area of similar size in the ocean with a similar amount of traffic suffers from a similar number of losses is, of course, irrelevant to the IT'S ALIENS OR WORSE "theories".

[Ilanthar]

By Walronate
The minor detail that pretty much every area of similar size in the ocean with a similar amount of traffic suffers from a similar number of losses is, of course, irrelevant to the IT'S ALIENS OR WORSE "theories".

So very true .