Calculating a position in advance, in open water

[Talbot]

for a distance of 300nm you could not use a transverse mecator chart, cause the answer would be wrong. You would be good using a gnomonic projection.

However ther is another way known to many users of norries tables i.e. using traverse tables.

 

[tome]

Quick short cut for d longtitude is to use the cosine of mean lat (37d 45m) and divide the departure (210M) by this which gives dlon of approx 265.6 mins = 4d25.6m

So your lon will be about 14deg 25.6mins rhumb

Just don't try this gross simplification in the survey world!

 

[BlueSkyNick]

that's what DJE's explanation is saying in a more theoretical way.

wont be using it in survey world - or even on passage, come to that! Was just sitting around wondering how far West we would end up by sailing NW from Gib for 2 days. /forums/images/graemlins/wink.gif

 

[tome]

Gets you to past magical 14 degs W at which point head for Oireland until past 44 deg N after which you can angle towards Ushant or S Brittany

 

[ChrisE]

Oh, if you'd said that I could have looked it up on Visual Passage Planner 2, a must have PC tool for all long distance sailors (really!).

 

[BlueSkyNick]

will look out for that - but wouldnt think of using it without understanding the the first principles /forums/images/graemlins/wink.gif

 

[ChrisE]

VPP2 user guide

Now Nick this is how it works, turn the PC on, click on the VPP2 icon, use mouse to move to spot on globe, read off poisiton, now that wasn't difficult was it?

 

[Twister_Ken]

NW from Gib

Sail NW from Gib and you'll very quickly bump into Spain. You need to go SW for about 15nm before altering course to the NW.

 

[BlueSkyNick]

Re: NW from Gib

pedant ! This is why I put a position in the initial question, which i then had to edit to avoid somebody telling me we would hit land first.

 

[onenyala]

Departure(East/West distance) = D.Long x Cos Mid Lat

 

[tugboat]

Re: NW from Gib

Have you tried playing with your Meridional Parts?

 

[LadyInBed]

This might be what you are after

 

[BlueSkyNick]

thanks Nigel,

DJE has put together an excellent little spreadsheet which does the sums based on whatever lat/long you put in to start and finish, or by bearing.

everybody is saying the same thing, mathematically, just different tools for working it out.

 

[tome]

Had a PM to remind me how terribly 2006 cosines have become - just mention it, don't mean to be bitchy (smile) but the New Year will soon be upon us and we don't want to be last year do we

Logarithmic haversines are the new tomorrow, and the King Edward geoid is gaining rapid favour over the 1936 Airey simple spheroid model. There are rumours that one of the original survey team participated in steam-powered events, so a clear and early reference to the Wake problem

Exciting times ahead, what?

Prize Question: What was the cost of steam (per HP) per 100 miles in 1939 at todays prices?

 

[DJE]

Spreadsheet Thread.