Course Heading - Beaufort to Bermuda

[scottmacc]

I posted this also in the Atlantic/Caribbean sectioin but since it pertains to navigation, I figured I'd post it here also...

Ok, I picked up some plotting sheets (always nice to learn something new), and also found a chart that covers the S.E. Coast of USA including the bahamas, Bermuda, and Geater Antilles. So my whole trip is on one sheet, plus I have all the local charts for my potential landfalls. Now I'm playing with actual courses. Using my GPS plotter, I plot a course from Marker #4 at the southern tip of Cape Lookout to S.W. Breaker at the southern tip of Hamilton Island,Bermuda. The GPS shows the heading as 110 degrees (true). When I plot the same course on the large chart, it shows the heading as 104 degrees (true). My GPS (garmin gpsmap 276c) is only using the base map not the chip for the Atlantic/Bermuda. Could the base map be this incorrect? Or, is a paper chart covering this large an area just not precise enough? I'd appreciate any of you with chart plotters letting me know what course heading you get between these two points. By the way, I'll be ordering the Garmin MUSO28R Bermuda chip as backup.

[Charlie]

Just a quick question not meant to be rude but on the paper chart did you make the adjustment for the yearly change in variation (or is it deviation?). A couple years of change on the age of the chart could account for the difference.

[scottmacc]

Not taken as rude....would be an easy mistake since I've gotten used to relying on gps for my lesser trips, however, This was just the true headings, not the magnetic. thanks

[captjohn360]

you have 5 things to concider . variation ,deviation,current,wind and your boats wind drift.its simple good luck. and now thanks to modern technology you can check and make course corrections with your gps as you go....jt

[Weyalan]

Long shot, but might be worth checking your GPS against a magnetic compass, just in case your GPS is set to indicate magnetic north instead of true north.

[mario f]

Var is 10 deg 38min W , your error is 6 deg- may be not a matter of magnetic or true reading.
Have you tried several other points to check and see if your error is constant ? say GrandBahama Is ? Might be human error, double check instrument position and technique etc.
Also chart datum...
Just my 2cents

[hellosailor]

IIRC the GPS will typically show you the course for a great circle route, the bearing will actually change over long distances as you are sailing the 3D arc over the earth's surface. The chart will only show you a simple Mercator course line (assuming it is the typical Mercator projection) and over long distances, the great circle route will be the shorter course.

There is nav software available (for the PC and even the Palm) that gives you options of initial courses to steer via different schemes (great circle, mercator, mid-latitude) and you'll see they differ depending on the start and end points, the directions, the distance.

[Lodesman]

scottmacc,

I think hellosailer is on the right track, but it wouldn't make sense for the GC track to be higher than the rhumb line (110 vice 104). If the rhumb line was 110, then an initial course on a GC might be 104 and that would increase as you got closer to the destination to a value greater than 110.
I think many GPS units allow the user to set GC or rhumb line as a preference in the set-up.
In this case I would look at the chart more closely. It might be a gnomonic projection, in which case a straight line is actually a GC. At the scale obviously required for such a large chunk of ocean, a mercator projection would certainly be distorted. I have a feeling it is another projection altogether, and is what is known as a 'planning chart', that is not intended for navigation at all.
I imagine the distance is about 500 nm, so there is not much advantage to a GC over a rhumb line.

Kevin