# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: True vs Magnetic
From: Lu Abel
Date: 1999 Jul 18, 11:19 AM

```This sounds like it could be the beginning of an interesting thread.  Never
being afraid to express an opinon, I'll start it.

As mentioned in a previous post, I've taught USPS coastal piloting courses
for 15 years.  The Power Squadron is purely "always plot and calculate in
true, then go through TVMDC"

While I absolutely agree with the need to do this offshore where one might
be going through changes in variation, everything I do for coastal piloting
is directly in Magnetic.

Why?  I think we'd all agree that the less pressure we put ourselves under
while underway the better.  The more of one's navigational work one
pre-calculates while at anchor or underway in fair conditions the better.

When I'm underway under rough conditions or have been sleepless, the last
thing I want to go through is TVMDC.  I don't care how long I've been doing
it, how instinctive it is, it's still math which is prone to error.  (And
the errors can be subtle, not in-your-face like many celestial
miscalculations)

So when I sail, all my course lines on coastal charts are marked in
magnetic (to be sure, I do put an "M" after the course in good USPS style
to make sure anyone else who reads the chart knows the direction is a
magnetic one).  That takes care of variation, there's still deviation.
I've found, on my boat at least, that I can adjust my compass to get
deviation to be no more than a degree.  At that point it's good but not
essential to correct for it -- being bounced around (or how did Dan put it
-- "buffeted") by weather or pushed by an irregular current or making
leeway can all affect course made good by much more than a degree.  Net
result is that the number I read off my preplotted course line is the
number I steer.  No math needed along the way.

courses in Magnetic.  After teaching everything in True, they note in an
appendix to their text that sometimes when navigating in a small coastal
area that marking courses in Magnetic may be more convenient.

As a side note, my favorite course-plotting tool is the Pocket Instant
Navigator.  The PIN consists of two plastic disks and a pointer all riveted
together so they can independently rotate.  The base disk has a 1/4" square
grid of N/S and E/W lines imprinted on it as well as a 0-360 degrees marked
around the periphery.  One drops the PIN over the starting point for a
course (the axle rivet is even hollow so one can see the point through the
PIN), aligns the base disk N/S by aligning the grid with the meridians and
parallels on the chart, spins the pointer to the destination (or along the
course line) and reads the course.

But wait, there's more!  In normal practice, one takes the second disk
(also imprinted 0-360 degrees, but with no grid), rotates it to the value
of variation, and then when one reads a course as described above one also
gets the magnetic course.  (Since I do most of my sailing in an area of
constant variation, I lock the two disks in position with a bit of Scotch
Tape).  In fact this "magnetic" disk has its degree scale outside the base
or "true" disk's, so the most obvious number under the pointer is the
Magnetic course.

Takes more time to explain it than to use it.  Bought mine many, many years
ago, don't know if it's even available any more, but it's absolutely my
favorite navigational tool.  Beats hell out of walking courses across to a
compass rose on a chart or using a traditional course plotter which gives
only true courses.

Lu Abel

PS - Does anyone know how to calculate leeway (for either power or sail)?
The Power Squadron asks one to use a leeway allowance in some its course
calculation problems, but it's always stated as "based on your experience
with your boat, you decide to allow 5 degrees port leeway."

At 07:39 AM 7/18/99 -0800, you wrote:
>Millard:
>
>I have seen a few small boat sailors and fishermen plot using Magnetic.
>But since day one I was taught to plot on charts or plotting sheets
>only true course. Then to do TVMDC for magnetic/CC. The reason being
>that variation will vary with your position and deviation with your
>[Snip]
>>He was working with True Course and I
>>had taken the variation from the charts and converted my course  to
>>magnetic.  When I asked him did he consider the variation, he realized we
>>had the same course only one was true and the other magnetic.  We set the
>>auto helm to the course  corresponding to the compass course, and to my
>>amazement Block Island, RI showed up a day and three quarter later.  We
>>never touched the helm until the entry into the Great Salt Pond at New
>>Harbor, Block Island, RI.
>
>Dan Hogan WA6PBY
>dhhogan@nav.cnchost.com
>http://nav.cnchost.com
>Catalina 27 "GACHA"
>
```
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