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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 05-03-2010
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a question about rhumb lines

I just came across the term "rhumb line" about half an hour ago (and actually that reminded me I was once at a nice restaurant by that name in Maine). The definitions I've been able to dig up on the internet say that it is a line that intersects meridians at a constant angle -- this I think I understand. However, just about every source goes on to say that it's what you get if you navigate a course on a constant compass heading. This does not make sense... wouldn't the magnetic variation of the compass cause you to not intersect every meridian at the same angle? You'd end up in some weird curving course, as far as I can intuit. If you had a magic compass that read true north (no matter where you are), perhaps you would get the course described...
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Old 05-03-2010
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You're absolutely right. A rhumb line across Casco Bay will be a constant compass heading, but one from New York to Ireland will change slightly as yo go along. I once learned all about the details of why that is so from the USPS's Junior Navigation course, but I have to confess that it escapes me now. I just remember it was so.
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Old 05-03-2010
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A rumb line is usually contrasted to a great circle route. The rumb line looks like a straight line between two points on a projection of the globe onto a 2 dimensional map. The great circle route is the straight line on a globe. In practice for short distances the two are virtually the same. For some routes, the great circle can be significantly shorter over great distances. The shortest distance between two points is a straight line, except of course when it must curve around the earth.
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Old 05-03-2010
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On a mercator projection map (most comun 2 dimension map) it will look like a straight line. On other projections, used for large scale maps it can appear like a slight curved line.

This is a Atlantic map (with a race going on):

Transat AG2R La Mondiale 2010


Click on the first square (on the top) to make it bigger and then click on the one that says "Orthodromie" (that's the word for rumb line on Latin languages) - the 6th from the left. The rumb line will be displaied.

"just about every source goes on to say that it's what you get if you navigate a course on a constant compass heading." Not a compass heading but a constant bearing.

Regards

Paulo

Last edited by PCP; 05-03-2010 at 05:00 PM.
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Old 05-03-2010
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Actually, a Mecator Projection is the only projection on which a Rhumb line is staight line. Magnetic variation has nothing to do with Rhumb lines. Rmeador both your definitions are correct but they are only applied to chart work. In practice you would have to take into account variation and deviation to remain on your charted line or Rhumb line. On longer east/west or west/east one uses a great circle route rather than a rhumb line because it is shorter. On a great circle route your angle to longitudes or course is constantly changing unless you are at the equator. A course on the equator is both a Rhumb line and a great circle.
Interestingly a GPS chart plotter will take you on a great circle route even though it would show a Rhumb line. If you leave NYC and put a go to way point at the English Channel the actual route you will follow will be a great circle.

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Quote:
Originally Posted by ebs001 View Post
Actually, a Mecator Projection is the only projection on which a Rhumb line is staight line. ...
Yes you are right. I was correcting my post, but you have posted while I was doing it.

Regards

Paulo
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Old 05-03-2010
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Quote:
Originally Posted by Hesper View Post
You're absolutely right. A rhumb line across Casco Bay will be a constant compass heading, but one from New York to Ireland will change slightly as yo go along. I once learned all about the details of why that is so from the USPS's Junior Navigation course, but I have to confess that it escapes me now. I just remember it was so.

Actually your magnetic heading is changing due to magnetic variation
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Quote:
Originally Posted by jerryrlitton View Post
Actually your magnetic heading is changing due to magnetic variation
I imagine Casco Bay isn't big enough to see the magnetic variation, well, vary, from one side to the other.
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Old 05-04-2010
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A rhumb line is defined as a course of constant bearing and as has been noted is a straight line on a Mercator chart. This makes them useful because they are quick and easy. However there are some limitations: they are not the shortest distance between two points, they are not suitable for high latitudes, over longer distances unless you are using a gyro then variation in magnetic declination may be an issue. Nice pic of that here btw (http://www.ngdc.noaa.gov/geomag/WMM/data/wmm-D05.pdf).
For distances of a few tens of miles in most navigable waters a rhumb is the same as a great circle. For longer legs you will probably want to start thinking in terms of great circles.
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Old 05-04-2010
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Quote:
Originally Posted by AdamLein View Post
I imagine Casco Bay isn't big enough to see the magnetic variation, well, vary, from one side to the other.
Adam, check out the chart (s) across the Atlantic. You will isogonic lines what are lines on a map connecting points of equal magnetic declination.

Jerry
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