How to depict atypical magnetic fields on a map

[Majere_Draven]

Hi- I'm currently (re)working on a global scale map. I just had a few small question, as I've never really done this before. I'm using CC3 to start- and will then turn over and trace the main shapes by hand. But first, There's an Idea I wanted to run by you guys.

I want a vein of magnetized cobalt in the sea that runs down an entire side of the planet. I want the magnetism to be strong enough to confuse compass's and lead ships astray to keep travel from going in that direction. Does that seem plausible, or should I just forgo the idea? If I keep it, How would I mark that on a map?

[sai]

That sounds . . . interesting.

If you think about it, only the people living on the opposing hemisphere will use North/South as the primary cardinal directions. Anyone within whatever range you decide on for the magnetic strip, will just adjust by having cobaltward replace North as the primary and they'll be able to navigate with few problems. In fact, if the magnetic field is particularly strong, cobaltward could almost completely replace North and all maps will be orientated away or toward that point.

However, if you still want it to be somewhere people avoid, it could skew the magnetosphere so that you get massive amounts of solar radiation in the area, depending on your planetary arrangement, and make travel in that area extremely dangerous.

Those are my thoughts. But it's your world. Go nuts.

[Hai-Etlik]

If you have a spherical planet, and it's possible to locate the stars or the sun, then people will be able to run along a parallel even if they have no compass.

Also I'm not a geologist or a physicist but I don't think it would be possible for that to work short of magic anyway, so you might as well just go with "magic fuddles the mind and all devices of navigation" rather than "magic created an enormous amount of cobalt and gave it a massive magnetic charge so that it can affect compasses thousands of metres away."

[ManOfSteel]

Unless...
there are a series of submerged islands in the sea, perhaps an entire mid-ocean ridge with massive iron deposits within granite domes. Not only are the currents between these barely submerged islands strong and treacherous, but when there are storms...and confluences of warm and cold currents in both the northern and southern hemispheres, as well as the tropical updrafts ensure that there are often storms... the sky is obscured and thus navigation by stars is impossible. Even worse, these terrible storms form within a matter of minutes and huge amounts of lightning are drawn to the underwater metallic deposits. The lighting generates sporadic magnetic fields which can make a compass useless for hours or even days.
I could imagine some ships encountering days of bad weather, and not seeing the stars for several nights, all the while battling storms at sea while trying to avoid the shoals. When the weather finally clears, they could be miles off course...if they haven't hit a rock or been struck by lightning. Lightning is attracted to the tallest object around you know, and those ships' masts are pretty high. Surviving a lightning storm wouldn't matter much if your sail was struck and burned. A ship off course, without a sail is not a good place to be. All you need do is examine one of the skeleton inhabited derelicts lodged on the rocks or read their logs describing their cowering below decks through night after night of lightning, fearing electrocution or fire, and their slow death from dehydration.
I'd be scared to venture into that area.