Re: Firmware bugs on 76CSx (and probably on all C*x models): No Great Circle routes!
- From: Lars <Lars@xxxxxxxx>
- Date: Fri, 28 Apr 2006 20:22:07 +0200
On Thu, 27 Apr 2006 18:35:07 GMT, "John R. Copeland"
<jcopelan@xxxxxxxxxxxxxxxxxxx> wrote:
The "direct" route you described sounds like a rhumb line.
I've been told that's used in ocean navigation,
It is indeed, but mostly just for short distances where the difference
in distance is nominal. This is mostly out of convenience since
rhumb lines are easy to plot on a flat chart.
For ocean passages great circles are often used to go the shortest
route. But exceptions are not uncommon, mostly then due to weather
conditions. A great circle from Lands End (English channel), to
Ambrose (New York), takes you to such high latitudes that during much
of the year you would run into heavy seas and arrive later than if you
had gone the longer rhumb line.
I doubt very much that a GPS would default to displaying rhumb line
routes since they are more complicated on a globe.
It is quite easy to check though. Create a route between two points on
the same latitude but far apart longitudewise. A rhumb line between
these will have a course of 90 or 270 degrees while the GC will
differ, and be banana shaped on a chart with correct angles.
The difference will be much more obvious on high latitudes and would
be none at all at the equator.
My testroute has these points.
Start point 60 degrees N and 15 East
End point 60 N and 90 East
A rhumb line will be Course 90 degrees, distance 2250 Nautical Miles
Great Circle will have an initial course of 56 degrees and be 2126 NM.
Obviously the compass course will change along the way as you pass the
longitudes.
If I create this route in my 69CSx it gives me the values I have
mentioned for great circle.
As an extra precaution I have tested these figures in competent
navigation programs.
Lars
Stockholm
.
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