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Thread: rhumb line placement

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      OldGuy is offline
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    Default rhumb line placement

    I guess I really don't understand cartouche (cartouches?). The navigation lines emanating from them must aid in navigation, but where do you place them in order for them to be useful? I've looked at many maps that use them and, although they look very nice and definitely add flavor to the map, they seem (to me) to be arbitrarily placed purely for aesthetic reasons. For them to be useful, it seems like there must be specific criteria that would dictate their placement. Does anyone know what that criteria might be?
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    Last edited by OldGuy; 02-09-2011 at 01:36 AM.

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      RobA is offline
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    Quote Originally Posted by aviation dictionary at Answers.com
    A line on the surface of the earth that cuts all meridians at the same angle. It appears as a curved line on the surface of a sphere. Only one such line may be drawn through any two points. Although this is not the shortest distance, the direction is constant. Normally, flights of less than 1,000 NM distance follow the rhumb-line track, whereas flights of a longer distance follow the great-circle track. Also called an equiangular spiral, loxodrome, loxodromic curve, or Mercator track.
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    As far as I can figure out, the origins of the rhumb lines are generally marked by wind roses having the cardinal directions, and are chosen to be easily identifiable navigable points... harbours or points where you could triangulate by line of sight.

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    Guild Master Gracious Donor Midgardsormr's Avatar
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    Clarification: A cartouche is a label enclosed by a border, a map's legend and/or title is usually on a cartouche. The rhumb lines may emanate from beneath a compass rose placed in a cartouche.

    On some maps, rhumb lines indicate bearing to a particular port or navigation point. The navigator plots the route from the rally point (from which the lines originate) to the destination port by simply aligning the heading along the rhumb line. Or they can run from rally point to rally point. This is an easy method of navigation requiring only a magnetic compass and an accurate chart.
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      OldGuy is offline
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    Ah, I see that I had my terms confused as well. But I guess I still don't understand where rhumb lines should originate in order to be useful. My poor understanding leads me to think that they would be most useful if each port had rhumb lines emanating from them. But I don't ever remember seeing a map that incorporated them in that way.
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    Last edited by OldGuy; 02-09-2011 at 01:37 AM.

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      waldronate is offline
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    Maps with many sets of radiating lines are called portolan charts. http://en.wikipedia.org/wiki/Portolan_chart has a reasonable description.

    Rhumb lines are hard to do on large-scale maps unless they are in a specific projection. This projection is called the Mercator Projection and it requires relatively advanced mathematics as shown at http://mathworld.wolfram.com/MercatorProjection.html.
    Last edited by waldronate; 07-10-2010 at 06:23 PM.

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      Ascension is offline
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    Since it's so complicated and I don't know the first thing about placing them properly I just made a large image of radiating lines that I can copy and just plop them down on my ports to fake the look.
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      OldGuy is offline
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    Quote Originally Posted by Ascension View Post
    Since it's so complicated and I don't know the first thing about placing them properly I just made a large image of radiating lines that I can copy and just plop them down on my ports to fake the look.
    All things considered, I think that method will probably work best for me as well.
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    Last edited by OldGuy; 02-09-2011 at 01:37 AM.

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      loongtim is offline
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    Yeah, I pretty much go by Ascension's methodology too. I started looking into this issue once and came across a coastal map of Africa that had eight sets of rhumb lines, even though most of the map was land, not sea. Looked completely random to me, so I figured most people who looked at my map wouldn't nitpick too much.
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      Ghostman is offline
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    The placement of the origins is not entirely arbitrary. They seem to be arranged in polygonal chains at equidistant intervals from each other. Like this:
    rhumb line placement-rhumblines.png
    Note that rhumb lines are only truly useful as a navigational aid on flat worlds - or a relatively small geographical area if the world is non-flat. The bearings they indicate on those medieval maps become increasingly erroneus the further you plot the course, due to the curvature of the planet's surface.

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