Great Circle or Rhum lines across the Atlantic

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waterboy

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Do any of the current crop of chart plotters calculate a great circle route for crossing oceans? Mine, a Navman 5500 seems to do the rhum line route only and thus would add at least 2 days passage, say between Antigua and the Solent. Alternatively, is it that hard to calculate a great circle route?
 
My GPS does a great circle, as do most AFIK.

If you want to work it out for yourself, buy a gnomonic chart. There is also a great circle plotting diagram but I doubt anyone still does it that way.
 
It is very rare to find a GPS that does anything but GCs, apart from very expensive ones. The way to test is to put in two waypoints of the same lat but diff long (don't use the equator) such as 50N 01W and 50N 10W and plot a route between the two. Starting from the first WP and the course comes up as 270, then it's a Rhum Line. If more than 270 then it's a GC.
 
I don't think you need either, the quickest way over is using the low/high pressures to your advantage or limiting the possibilities of no wind or too much wind. If you're in a moboat then you might consider it but even then I'd be avoiding some weather systems. My experience both going East and West is that the weather systems decide your route not the GPS. (we managed a 14 day crossing in a Sweden 38 in 94)
 
You can use the 'Haversine' formula, or ABC tables in 'Norries' to calculate a GC, but its easier with a gnomic chart and pick off your days run as waypoints before transferring to a Mercator chart.
I would have thought weather would be more of factor in a sailing vessel on an Atlantic crossing.
 
ABC / Norries or Burtons ...

This is the old hat way and suffers from the angle to sail becoming distorted as you get closer to destination. Because as we used to do - you calculate a total GC distance and initial course. This course you then keep till next noon - calculate next initial course etc. This actually is a less accurate way of doing it - but most practical.
I found out about this factor - when I bought my Nav Calculator which actually calculated ALL intermediary points of a GC in the first calculation set by longitudinal steps YOU decide. This then produced a more accurate distance to go based on the legs and not the overall GC distorted by Noon to Noon. (Maybe my explanation runs away with words ....)

Crossing N.Atlantic especially from Western App's to say Ambrose light as example - the northing is too high in most seasons and it is wiser to complete a Limiting Latitude Composite GC ... This is what the Liners used ...

The Calculation of GPS can be checked very easily .... compare a Mercator calculatiopn with your GPS data ... A simple program such as MNAVD1.exe ... freely available on web will give you all the calculation routines you need ... GC, Mercator etc. It has a load of stuff that actually you will probably never use .... but it is simple, small and installs with no registry or other OS gimmicks etc.

My Ti59 Programmable Nav Calculator is NOT for sale before anyone asks ... It is one of the best ever Nav Calculators ever ...
 
Ah, the unforgotten pleasures of 'Chart Projection Theory'! Where's me Polar Stereographic, me Lambert's Conformal and me Azimuthal Equidistant......?

Recalls a conversation snippet heard amongst a group of OSTAR veterans - " Did you use the Great Circle Route or did you just go straight there?"



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You can download a free version of Netpass Distance
its what ships use to plan their passages. it plots shortest routes with all the waypoints between any ports in the world.

Portsmouth to Antigua is 3534 NM and only about 60NM further by rhumb line, but since either rout taks you within 50Nm of the Azores, isnt it a bit academic
 
I'm with you there Tiga. The quickest way across an ocean for a sailing boat is never a great circle. It's always at least a dogleg to take advantage of wind systems and avoid calms/storms. Great circles are for commercial ships and classrooms.
 
Hi Barnicle!

""Portsmouth to Antigua is 3534 NM and only about 60NM further by rhumb line, but since either rout taks you within 50Nm of the Azores, isnt it a bit academic ""

Entirely agree. There's so much rubbish talked about this. Having said that, before we did our first circuit, we dedicatedly checked all the figures a couple of times, sat back, and burst out laughing when we realised just what the differences were! /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

And that assumed we could go in a straight line. I remember checking the difference between the two 'routes' years ago before leaving the Cape Verdes. Wow, a whole 4 miles or so as I recall. Required serious thinking about provisions, water, etc etc!! /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif
 
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