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


Thread: a question about rhumb lines Reply to Thread
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Topic Review (Newest First)
05-04-2010 10:42 PM
jerryrlitton 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.
Quote:
Originally Posted by AdamLein View Post
Casco Bay is not the Atlantic Ocean. As far as I can tell either from isogonic charts (which are just about the smallest scale charts out there) or from the roses on Chart 13290, the variation does not change significantly throughout the bay, being 16°00'W for most of the bay and 16°15'W in the northern, almost inland parts. 15' is not relevant at those distances.
However go from NY to Ireland the Atlantic is in the way no? RTFT.
05-04-2010 11:12 AM
AdamLein Casco Bay is not the Atlantic Ocean. As far as I can tell either from isogonic charts (which are just about the smallest scale charts out there) or from the roses on Chart 13290, the variation does not change significantly throughout the bay, being 16°00'W for most of the bay and 16°15'W in the northern, almost inland parts. 15' is not relevant at those distances.
05-04-2010 09:03 AM
jerryrlitton
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
05-04-2010 08:29 AM
Broomfondle 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.
05-04-2010 12:38 AM
AdamLein
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.
05-04-2010 12:03 AM
jerryrlitton
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
05-03-2010 06:04 PM
PCP
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
05-03-2010 05:57 PM
ebs001 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.
05-03-2010 05:39 PM
PCP 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
05-03-2010 05:05 PM
noreault 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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