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Re: Great Circle Course via calculator & HO 208
From: Stan K
Date: 2016 Oct 7, 20:50 -0400

What Gary describes is one of two methods for creating a "composite route", what is referred to as Method 1 in the following paragraph:

A great circle route can reach latitudes higher than that of the point of departure or destination. To avoid high latitudes where ice or gales might be

encountered, a modification of great circle sailing called "composite sailing" may be used. There are two methods used to accomplish this. In Method 1,

a great circle route is plotted from the point of departure to the destination and is followed from the point of departure until it crosses the limiting

latitude, then the parallel of the limiting latitude is followed until the great circle again crosses it, then the great circle route is followed to the

destination. In Method 2, one great circle is plotted from the point of departure tangent to the limiting parallel, and a second from the destination

tangent to the limiting parallel. The first is followed from the point of departure to the limiting latitude, then the parallel of the limiting latitude is followed

until the second great circle reaches it, then the second great circle is sailed to the destination. Method 1 is easier to plot, but Method 2 results in a

shorter course. (In both methods, plotting is done on a gnomonic projection.)

Picking some random departure and destination points, departure 41ºN 72ºW, destination 41ºN 0ºW/E, here are the numbers, with the composite route limited to 44ºN, a randomly selected value:

Full great circle 3160.1 nm

Composite Method 1 3198.8 nm

Composite Method 2 3181.6 nm

Probably not a great example, as Method 1 is only 17.2 nm (about a half a percent) longer than Method 2 (and only 38.7 nm, or about 1.2%, longer than the full great circle), but it makes the point, if only an academic point.

Stan

-----Original Message-----
From: Gary LaPook <NoReply_LaPook@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Thu, Oct 6, 2016 5:34 pm
Subject: [NavList] Re: Great Circle Course via calculator & HO 208

Then you fly a "composite route." Lay off the great circle, on a globe, gnomic chart, or, last resort, compute it. See if there are dangers or travelling into high latitudes. Then plot the points where the GC crosses that limiting latitude and then sail the GC to the first point, a rhumb line between those points and then the GC for the rest of the voyage.

From: Bill B <NoReply_BillB@fer3.com>
To: garylapook---.net
Sent: Thursday, October 6, 2016 2:03 PM
Subject: [NavList] Re: Great Circle Course via calculator & HO 208

On 10/6/2016 4:08 PM, Frank Reed wrote:
> "I guess there could also be yet another kink around eastern Ukraine as
> well."

In the marine world, they may also  be good reason other than weather to
not travel a great-circle route. Icebergs for one.

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