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

[ladiestorm]

I don't know if this will help me make better maps, but it's something about map making/using, that I've never understood, so I thought I would ask.

I've seen a lot of old world maps (and even some of the maps here on this site) that use the navigation lines. Now, I understand about Latitude and Longitude, those are the navigational lines based on the poles and the equator. Those lines create coordinates that help you find a specific point on a map/globe. That part I understand.

But I have seen a lot of maps that have navigational lines that start in places other than the poles and the equator. http://www.cartographersguild.com/sh...539#post299539

This is an example of what I'm talking about. This mapper has 3 separate points of origin for navigational lines.(sorry to hijack the thread but I wanted an example of what I'm asking about)

I want to understand why this is done. I realize there is a reason, I just have no clue as to why. I don't really understand navigational lines beyond latitude and longitude, and I want to understand.

So here are my questions. Why use origin points beyond latitude and longitude? What do these other origin points represent? How and why would a map maker use more than the poles and the equator for navigational lines? I also notice that sometimes the poles and equator are NOT used, and other origin points are used instead. What would be the reasoning behind this?

Any assistance in furthering my map making education would be greatly appreciated!

[Falconius]

Rhumb lines. They are used as a navigation aid, they indicate a line of constant bearing one can follow.

Here at the guild though they are mostly used for decoration.

[ladiestorm]

rhumb lines? Pardon my ignorance, but what are rhumb lines? How are they different from latitude and longitude? How does one determine where they start?

[Falconius]

I don't really understand them too well myself. What they allow is for people to follow a point on a compass without having to change direction. If they followed that point all the way they eventually end up at south or north pole depending on which way you are going(the other option is to use a straight circle (like a slice through the earth) at which point you can end up where you started, but for this you need to constantly adjust your heading). The nodes you see where these lines gather is where that line is true to on the map (as a constant bearing). Past that node the bearings change (ie if you set your bearing on a previous rhumb line it would no longer represent a constant bearing and you'd have to adjust to keep going straight). These rhumb lines are only straight on Mercator projections navigation of course being the main reason for developing Mercator. On other map projections they'd have to be curved slightly to remain true.

Here's the wiki link, but I think it does a poor job of explaining it.

[waldronate]

See https://en.wikipedia.org/wiki/Portolan_chart for a discussion. The basic idea is that you want to be able to navigate using a compass, a chart, and some basic tools like a divider for marking distance and a tool for extending a line (or making parallel lines). A rhumb line is a line of constant compass bearing; on a Mercator projection, it is conveniently a straight line. The networks of rhumb lines that you often see on old charts are conveniences for navigation.

One way to use such a compass rose element is to place the center near or on an area that you commonly navigate from. Then you can get out a string and some tacks to plot a course and follow your compass to the destination (assuming an appropriate chart projection and navigation area). Another way you're likely to see them is as a set of 8 or so roses around the edge of the chart (often pre-printed). During exploration of an uncharted area via ship, it is then "easy" to use a compass and distance to map out the coastlines. The multiple roses make it quicker and less error-prone to mark out the correct angles. Navigational aids such as a sextant make finding latitude simple enough for correction, but it took quite a while to get accurate enough clocks to work out longitude for navigational purposes.

As Falconius points out, most folks here don't use rhumb line networks for anything more technical than decoration.

[Tonnichiwa]

This is gonna be hard for me to explain. I used to do it for real when I was in the Navy but I still can't explain it very well. Basically there are two different kinds of compass. One is true north and represents where north is for real on a map. The second one is called relative north. Ocean navigation maps are usually really big and in order to figure out relative north on a map you need to use a protractor that extends. If you were trying to figure out relative north on a map that only had one set of Rhumb Lines (navigation lines), that were, say, on the other side of the map from where you were doing the calculation of where your ship is, then you would have to drag your protractor a very long distance across the map, increasing the chance that you could mess up your position on the map because your protractor may move slightly after a few times of extending it. Those little mess ups can majorly impact your position on a map. So Rhumb Lines were used in multiple places on a map so a person didn't have to drag their measuring equipment half way across the map. They could just go to the closest compass and go from there.

EDIT: DOH!..ninja'd

[ladiestorm]

okay... this is all starting to help me understand the need for so many origin points. Thinking about it after the explanations, it actually makes sense, sort of. But it also leads to other questions, such as, Tonnichiwa(and I was hoping you would join in on this... I knew, being ex Navy, you would probably be able to help me with this) you mentioned that there are two Norths, true North(which, I assume, originates at the North Pole) and relative North.

The obvious follow up question, would be... what's the difference? Why do you need a relative north? What purpose does it serve?

But I begin to understand, I think, the idea of different origins for rhumb lines (why are they called rhumb lines? why not rose lines? It is the compass rose, after all). For example, if one wanted to plot quickest(or easiest) route from say.... Sidney, Australia to... say the nearest port to Dublin, Ireland - they might set a series of rhumb lines with origin points at all the ports they would have to hit to make that voyage? One starting at the port closest to Sidney, then maybe setting new rhumb lines from the ports they would have to hit to resupply in to keep them on the correct bearings for Ireland? And of course, they may have to change direction at some point, so they would need to know at which point they would need to change... the crossing of rhumb lines from a different area perhaps?

[Azélor]

There is the true north located in the north pole and the magnetic north located somewhere in the Arctic ocean. There are also two poles in the south, just like in the north. Magnetic poles are always moving. The true poles are not since the axial tilt of the Earth is very stable thanks (in large part) to the gravitational pull of the Moon.

And your sailors would also have to consider the direction of the winds and water current s if then want to get there faster.

[ladiestorm]

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?

[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.