In Creating a Route on Charts, Do You Use True or Magnetic ?

[boatman61]

LOL... I think I said that 23 posts ago Barnie.... begining to feel like its .... cant get through

[barnakiel]

Gimme 5!

;-)

[Lodesman]

Originally Posted by barnakiel

PLEASE NOTE dear sailors or pretenders (hahaha) that the T and M are THE SAME LINE on the chart.

I don't think anyone has suggested otherwise; I inferred from the OP that the question was to do with labelling the lines. The OP specifically asked about route plotting; the other part of chartwork is fixing. There is no advantage to labelling the lines in M; you still need to convert to C to steer the course, so you might as well label the C course. If you label the T course as well then the conversion is plain to see on every heading in the event you wish to plot a LOP. Example: if the track is labelled 345T/350C then you subtract 5 from each compass bearing before you plot the line on the chart using T bearings to line up with the compass rose or the grid. The inner magnetic rose is historic, so is typically useless.

[barnakiel]

Originally Posted by Lodesman

The inner magnetic rose is historic, so is typically useless.

Agreed with all your post, except the 'useless' word.

I think we use it as a starting point to calculate the actual var - historical (from the rose) + annual corrections = actual. Off course, we can get the actual var from other sources too.

BTW - did you know in the old times they used var as a way of finding their position in ocean crossings? I believe this was done by cross referencing with the sun (?).

b.

[bewitched]

Originally Posted by Lodesman

I inferred from the OP that the question was to do with labelling the lines.

That's what I understood.

And for my simple mind, keeping everything on the chart as True keeps life simple. It tells me where I am and where I want to get to.

All the calculations that tell me which way to point the boat to follow that line on the chart are done by my instruments. (which because of weather and current information, rarely tell me to follow the rhumb line in any case).

But, as I'm often informed on this forum, one day my instruments will be struck by lightning and so I'd have to revert to pencil and paper. In this case what would go on the chart?

True - yes - tells me where I want to go

Magnetic - no - would just confuse me and doesn't give me any additional information above True - which I already have

Compass - no - Is of no value to me - it doesn't tell me where I am or which way to point the boat

Heading - possibly - because this tells me which way to point the boat. But, more likely, I'd scribble out the calcs for TVMDC and Leeway in the log or something and then write the heading on the compass binacle.

What is on the chart is true

Where I need to point the boat (allowing for variation, deviation and leeway) is beside the compass.

[lordgeoff]

I am a late commer to navigation and just love the combination of GPS and Autohelm. I double check everything on the paper charts and also pencil in my position on the paper chart. Also I carry a few spare GPSs which I double check against the boat GPS. One of the comments above is the plotter (I use a lap top) does all sorts of great information things. I agree. I check there is nothing hard, sharp or shallow within 6 hours sailing before I hand over to my wife and have a kip for an hour or so.

[lordgeoff]

Oh I also keep my laptop and spare GPSs in the oven when there is lightening about. I'm told the Faraday Principle should protect them from lightning. The biggest danger is not taking them out before cooking a potato bake for dinner.

[SimonV]

I have Just read all of this thread and It would seem most of you agree with Me , the chart is TRUE, you plan TRUE, you plot TRUE, set the Chart Plotter TRUE, and as I cant steer better than + or - 20-30 degs in a seaway on the compass, I guess I tend to treat the compass as sort of, nearly, not quite true. At the end of the day they are all just numbers and if as you should, plot your course every hour or 5 or 12 and then you make adjustments accordinly.

[Lodesman]

Originally Posted by bewitched

What is on the chart is true

Where I need to point the boat (allowing for variation, deviation and leeway) is beside the compass.

Exactly! In your previous post you made the point that Magnetic was a "theoretical" point between the useable Compass and True - I totally agree with that.

For Barnie I'll clarify my "useless" comment. The Magnetic inner rose is not useless per se; but it does suffer serious drawbacks to its usefulness. First, there's the annual change to variation that has to be calculated - yes this will be constant during the trip, but the rose will be off by seconds, minutes or degrees necessitating a correction with each use. Second, the roses tend to be muted in colour - it's difficult enough using the outer ring, so why would anyone want to make it more difficult by using the smaller inner ring. And the third strike - the compass roses on a chart tend to be located in out-of-the-way areas of the chart, so even with a full-size chart table there can be a lot of rolling and sliding to get to a rose; with a square protractor you've got a compass rose anywhere there's a parallel or a meridian - but it's only in True. All you need is a protractor and a straight edge to easily and quickly measure bearings in the actual location of your chartwork - way easier on a small boat.

[Dromomania]

Personally unless I'm offshore I navigate mostly off ranges and known points to get from a-b and use my speed over ground to determine where on my "line of position" I am. For offshore I plot everything in true and keep a "corrected course list" on the binnacle and steer off the compass. Crude...yeah ...but it works for me .........

[sneuman]

Originally Posted by SimonV

I have Just read all of this thread and It would seem most of you agree with Me , the chart is TRUE, you plan TRUE, you plot TRUE, set the Chart Plotter TRUE, and as I cant steer better than + or - 20-30 degs in a seaway on the compass, I guess I tend to treat the compass as sort of, nearly, not quite true. At the end of the day they are all just numbers and if as you should, plot your course every hour or 5 or 12 and then you make adjustments accordinly.

Well, in that case, you could do the same with M and be steering something closer to an accurate course.

[CaptnPete]

I just read this article published in The News-Press about the use of nautical charts and compass :
Title : Boating, learn to use nautical charts and compass for navigation

Peter

[lordgeoff]

Originally Posted by sneuman

Well, in that case, you could do the same with M and be steering something closer to an accurate course.

True. Talking Passages.......GPS and Auto Helm and compare the actual course achieved with the intended course each hour. Regardless of Magnetic or True the currents are going to vary your course anyway.

[jim_thomsen]

I was taught to use Mag in my USA course and to only use True in my RYA UK course.
I now use only true, since some charts by some countries do not have a magnetic rose printed and others have one that was from the 1980's, so you then need to make the yearly corrections anyway.
After plotting on a paper charts I write both T and M on the line.
My Raymarine plotter changes from mag to true easily.

[sneuman]

I just happened to see a video by John Rousmaniere. He says magnetic "to avoid confusion." I haven't had a chance to check, but I assume he says the same thing in "Annapolis Book of Seamanship".