True or Magnetic

Frontier

Frontier

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I have got myself a new GPS/Plotter and it can be set to True or Magnetic. So for costal cruising, Channel-Solent-west country etc, do I really need to bother about all the maths converting True to Mag. Isnt it easier to just set it to Mag?

Bet I will regret asking this /forums/images/graemlins/smile.gif
 
Wouldn't it be easier to set it to True? That's what the charts are anyway so makes it easy to draw your plots (you do use paper backups don't you? /forums/images/graemlins/smile.gif )
 
Always use true myself. It's the compass that has the error I think it's always easier to think of the compass reading needing correction.
 
I am a dissenting voice. I prefer to have all items that provide a bearing, using the same standard (i.e. magnetic) Then you dont have to think which one is true/which magnetic.
 
Yes I always set to true because I always use paper charts and a gps/plotter as backup. I have heard it said many times that most humans cannot steer a course to within 5 degrees! so with the average mobo it almost never matters! mind you an auto pilot should do better.
However I feel it far more important for the sailing skipper, because his boat is affected far more by the winds and currents as well as being several degrees out by not paying attention to the difference between True or Magnetic.

Barry
 
I'm on your side. The main navigational kit is the No1 steering compass which is magnetic. If anyone takes a bearing (by that compass or the h/b) it's magnetic. GPS can be configured to talk magnetic so if you give the helm a bearing to the mark it's likely to be correct. The radar (if configured so) can only rely on a fluxgate compass to give north up so it's head up on my boat. I use a Breton plotter which is easy to configure for magnetic.

We can all convert from one to the other but when it's cold, wet, windy (or foggy) and you're tired I prefer to reduce the risk of error.

If we were big enough to run a gyro then that would be a totally different kettle of fish !!!!
 
I always work in true but I think it depends somewhat on how one drives the boat.

If one usually uses the autopilot (here assuming not just autopiloting between waypoints) then the easiest is to just turn the boat so that the heading vector on the chart display extends to the next turning point (eg into a channel, around a headland, or whatever) make allowance for drift further down the route if it is expected to be different than at your present position and the boat is slow or the sector long and go for it (this may be a bit easier for me too as I use an ECS with a big screen).

If one is manually steering the boat to a compass course against a magnetic steering compass then I suspect working in magnetic might be easier for many.

As for taking bearings for a fix, I am not sure why one would want to take them when navigating with plotter/ECS, especially in a fast powerboat as yours might be. I find that the 10m or so accuracy of the GPS is far more than I could ever dream of getting from my own bearings. If the GPS is faulty then it is likely to be wrong, wrong, wrong or dead.

And to check if that is so, what I do is just maintain spatial awareness of where I am counting off landmarks if any to be seen. Then I periodically look at the display and think "OK that rock should be about 30 degrees off port bow" and see if it is roughly so. If I want a more accurate check I lay a bearing line over the landmark (or landmarks) on the display to give the bearing off the boat's heading and check that across the steering compass (easiest as no real adding/subtracting involved as can just count off the degrees around the card from the indicated heading) or with handheld (harder as adding or subtracting the bearing from the heading required).

Variation then does not come into it at all, with the main error being the boat maintaining its heading during the process which is never a problem. I emphasise that these are just meant as checks on the integrity of the GPS positioning and not meant to be fixes themselves.

By the way, variation here is over 20 degrees, not the 5 or whatever as in UK, so may also account for not wanting to get involved with magnetic.

John
 
But you would still need to apply deviation, and during the course of a day's sail E-W here the variation changes about a degree, so I wouldn't want to keep changing the gps settings.

Personally I log the compass reading whenever I log the track, so if it all goes down I can keep on that track (until the tide changes of course!)
 
Chart, pencil ,rubber, Bretton Plotter, or similar,convert to true and C.T.S. Why? Long Post (with giggles) about a recent trip will explain shortly when time permits! Magnetive may seem attractive but base everything on the Truth /forums/images/graemlins/smirk.gif
 
Absolutely. You don't have the option of setting to Compass since the GPS has no idea of the deviation. And in fact if you have more than 1 fixed compass they may very well have different deviation. Hence True or Mag makes no difference with respect to the compass. You still need to do an extra conversion.

So the choice has to be True so it's easy to work on the chart.
 
Many plotters PC and Fixed I've seen can display both true and Mag.

In fact some PC based plotters you can even make up dev'n tables for it to use.

Me ? I have Mag and true on PC plotting, True on GPS Plotter.
 
I'm with Talbot - Even before GPS I always navigated - piloted in Magnetic - All the information coming in is in magnetic - steering compass, handbearing compass, in the days of radio beacons - magnetic and nowadays I always have the GPS (s) set to magnetic and if I draw lines on charts I take them off the magnetic rose.

I have always swung my steering compass so that the deviation is so small as to not be worth worrying about which means effectively there are no adjustments to be made which really helps when you are tired and everything is bouncing around.

In big ships almost all the information is coming as true - the steering compass is in true the pelorus works in true for the bearings etc. I think the RYA followed this big ship system in their desire to be PC.

I agree with John that when in areas with huge variation it makes sense to do it in True but I seem to remember in his waters and the 'Bermuda Triangle' area still doing it in Magnetic... Not sure there is a 'right' way but I prefer to keep things as simple as possible

Michael
 
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