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Old 08-03-2015, 21:23   #1
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True or Compas

I have always ploted using True but I have recently had a few conversations
Where a few said no plot in magnetic,
Are UPS still used
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Old 08-03-2015, 21:29   #2
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Re: True or Compas

If you are plotting a course long enough to change variation zones plot true.

Otherwise plot magnetic and save a step.
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Old 09-03-2015, 04:43   #3
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Re: True or Compas

Hi forgive my ignorance but as the chart is drawn with ref to true drawing lines on it that are magnetic is nonsense
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Old 09-03-2015, 05:24   #4
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Re: True or Compas

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Originally Posted by dnickj View Post
Hi forgive my ignorance but as the chart is drawn with ref to true drawing lines on it that are magnetic is nonsense
Not quite correct. A proper chart will display magnetic variation along with the annual rate of change of the variation. Many charts will even show the different variations for different area's of the chart. Look closely at the the True North of the compass rose on the chart it will have a variation line showing magnetic north nearby.

The other thing to remember is that a magnetic bearing and a (magnetic) compass bearing are not usually one and the same. The bearing your compass displays will usually have some degree of deviation error. Deviation is the error in the compass caused by it's proximity to other magnetic fields or just bad alignment.

The usual method is to plot true bearing and then use the variation (from the chart) and deviation (from a deviation - error - card for the actual compass) and use these values to calculate the compass bearing required. Also, don't forget that the effects of drift will also need to be factored as well at times.

Of course, in this day and age of push button navigation, this is all almost as irrelevant as using a sextant to most navigators.
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Old 09-03-2015, 05:31   #5
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Re: True or Compas

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Hi forgive my ignorance but as the chart is drawn with ref to true drawing lines on it that are magnetic is nonsense
Obviously you have not seen that your chart also has a circular thingy somewhere showing Magnetic north and little numbers from 0 to 359..
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Old 09-03-2015, 05:37   #6
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Re: True or Compas

Yes I have noticed the compass rose and that is what i use to covert from true to magnetic but as I said as the chart is drawn in true if I then place my course line in it in magnetic it will be out by the value of variation at the time
Surely course line should be plotted in relation to the drawn chart the variation and deviation applied or am I just being stupid
But in reality I use software to plot my courses to steer
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Old 09-03-2015, 05:38   #7
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Re: True or Compas

By Knowing the variation and the deviation it is simple enough to then convert to compass, magnetic and true.
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Old 09-03-2015, 05:45   #8
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Re: True or Compas

Keep it standard and plot in true. Getting fancy and changing all the rules is just going to confuse you. You might know that you like to plot in magnetic or compas- but nobody else will know that and will just assume you do what everyone else does and plot in true.

The inner rose can't be plotted off. The inner rose is to assist you in conversion. The outer rose is always correct because it never changes. The inner rose is based on a moving magnetic pole. If you look at your at the inner rose it even tells you how much it changes. A 20 year old chart will easily have the inner rose out by a degree, where a 100 year old chart will have accurate true.

Your electronics will all be oriented to true, even your compas isn't oriented to magnetic, due to deviation. There is no advantage to plotting to magnetic.

All formal navigation studies, whether they be recreational or prifessional plot with true because it's a constant and it's how your chart is oriented.

If you decide to go against the grain and plot in magnetic or compas make a giant sign for over your chart table so your crew has a chance of figuring out what you've done.

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Old 09-03-2015, 05:47   #9
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Re: True or Compas

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Originally Posted by dnickj View Post
Yes I have noticed the compass rose and that is what i use to covert from true to magnetic but as I said as the chart is drawn in true if I then place my course line in it in magnetic it will be out by the value of variation at the time
Surely course line should be plotted in relation to the drawn chart the variation and deviation applied or am I just being stupid
But in reality I use software to plot my courses to steer
The lines of longitude on the chart are aligned with true north. The magnetic rose is aligned with magnetic north for that area. The proper method to plot a course...

- Draw the course line
- Use a protractor to measure the course angle (True Course) referenced from the longitudinal line (0*)
- Apply East/West variation to get Magnetic Course
- Apply compass deviation to get magnetic Heading (CTS)

The short cut is to draw course and derive the course angle with a set of parallel plotters off the rose, deriving Magnetic course immediately.

If you are measuring magnetic course off the rose and then adding/subtracting variation, you are doing it wrong.
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Old 09-03-2015, 05:48   #10
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Re: True or Compas

True for planning, magnetic for steering.
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Old 09-03-2015, 05:52   #11
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Re: True or Compas

You can plot whatever you like on a chart. The convention is to proceed your degrees notation with either (T), (M) or (C). Sometimes it is beneficial to plot a position or bearing in degrees other than true, but for planning a route True is the da boss.
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Old 09-03-2015, 05:54   #12
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Re: True or Compas

Yes true but I find it more time consuming paralleling off the compass rose more long winded than just doing the calc
And it assumes a recent chart with no significant change in the variation - I believe the op was discussing it as being easier and quicker
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Old 09-03-2015, 05:56   #13
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Re: True or Compas

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Yes I have noticed the compass rose and that is what i use to covert from true to magnetic but as I said as the chart is drawn in true if I then place my course line in it in magnetic it will be out by the value of variation at the time
Mate, are you trolling or is this for real? When using the magnetic rose you adjust up or down a degree or two to reflect the yearly *minutes* of variation change since the reference year. If you sail in teh same area all the time you will know the adjustment it by heart and it will become automatic. You will only need to change it by a minute a very few years, just like you change your reading glasses.

Quote:
Originally Posted by dnickj View Post
Surely course line should be plotted in relation to the drawn chart the variation and deviation applied or am I just being stupid
But in reality I use software to plot my courses to steer
Yo are confused. The line you plot/draw is a set of points that have their own lat/lon, which is neither magnetic nor true. The process of plotting the line can be executed with the same outcome by either using:
  1. true North and angles (courses) to True North ( a bit longer if you use a real compass, but will work for a long passage despite differences in variation along the passage)
  2. magnetic North and angles (courses) to magnetic North (faster if you use a real magnetic compass, but does not work well if you are dealing with a large area in which there are significant changes in variation)
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Old 09-03-2015, 05:57   #14
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Re: True or Compas

I'm starting to suspect that maybe the OP is talking ECS plotting. In this case you do whatever takes your fancy because the numbers on the steering compass are all but meaningless, as one just follows the highway on the plotter.
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Old 09-03-2015, 06:04   #15
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Re: True or Compas

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True for planning, magnetic for steering.
Magnetic for steering doesn't make sense. You're steering off your compas (C).

There is no good reason to put a magnetic course any where except for in the middle of your TVMDC conversion. Write your true on the course line with a T and if you want a compas course write it down and mark it with a C.


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