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?

[Tonnichiwa]

Like I said, this is very hard for me to explain. They need to have relative north and true north so they can triangulate your position on the map using trigonometry. A Navigator needs to know the course, speed, and bearing of his ship, and when near others, try to figure out the course, speed, and bearing of other ships as well. Now if they were planning to go somewhere like from Australia to Ireland, they wouldn't necessarily plan out the whole trip the way you describe because there are so many variables involved that are constantly changing. Like magnetic north, the currents in the ocean, the weather conditions, pirate threats, unfriendly naval powers, etc. Any of these, or even combinations of these can cause you to change course drastically and so you basically kept an idea of where magnetic north was at all times so you could try to chart your position from there. Sometimes, especially if you got lost in a storm, the only way to find your bearing again would be through magnetic north. that gave you an idea of what direction you were going. So often they would only plan the journey for as far as they can go on a certain regional map. Then they pull out the next map and do it again. Maps of entire worlds are hard to navigate because of the distances involved. Regional maps can get more accurate readings of where you are because they one inch doesn't equal 500 miles necessarily. So minor faults in navigation would be catastrophic on a world map, because if you are off in your measurement by a tiny bit it could be hundreds of miles, but regional maps are better, because the distances are smaller.

Now trying to tell you HOW to figure out your course, speed, and bearing is the hard part for me. Mainly because I suck at trigonometry. Effectively, you draw a line with minor changes of position along that line called a "tac". you try to follow the line as close as possible adjusting your heading every 15 minutes or so. And the whole time you are calculating and re-calculating by figuring out your relative position compared to your last known point. Ugh...so hard to describe...I wish I could just show you

So the rhumb lines are mainly there for your tools, not necessarily as a guide to where you are. It gives you a parallel line to access when using your protractor. You would set your protractor along the rhumb line, snap it out twice, and then draw your line. ...agh,....this is probably just confusing you. Maybe the best way to find out would be to look up how it's done on Wiki-how or something. It's been 26 years since I've had to do this..haha.