# NavList:

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

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Re: SS61 course to steer
From: Jan Olav Mossik
Date: 2000 Apr 09, 04:33 EDT

```On 8 Apr 2000, at 01:29, Dan Hogan wrote:
>It's:
>        T(ure Course)
>        V(ariation +/-)
>        M(agnetic Course)
>        D(evation +/-)
>        C(ompass Course)
>
>        Add easterly variation/deviation when un-correcting (True-to-
>        Compass)
>
>        Subtract easterly variation/deviation when correcting (Compass
>        Course-to-True)
>
>
>To do it mathematically is a kind of long explanation. Most
>Navigation manuals call it the current triangle and explain it with
>graphic vectors. Bowditch explaines current using vectors.
>
1. I do not agree in the way var/dev correction is explained, it is very
easy to misunderstand (in fact it is wrong), therefor;
The rule of thumb is (please take no offence) West is best (so therefor add)
East is least (so subtract).
When going form the map to the compass, opposite when going back.
It can be mathematically proven by using the formula for course reduction.
True track - (minus) variation = magnetic track, here to remember that - -
(minus minus) gives + (plus), west variation has - (minus) sign due to the
fact that in this situation the magnetic pole is to the left of the true
north pole.
2. To calculate the correction angle for tidal stream one can use a modified
wind correction formula found in the nav.zip
CA=ASIN(SIN(TSD-TT)*(TSS/BS)    for use with a calculator
CA=ASIN(SIN((TSD-TT)*PI()/180)*(TSS/BS))*180/PI())    for use with a
excel sheet
CA= Correction Angle left or right (- or +)
TSD= Tidal Stream Direction in the theorem of the direction it is coming
from (opposite to normal tidal theorem)
TT= True Track
TSS= Tidal Stream Speed in knots
BS= Boat Speed in knots
With regards Jan :-)
---------------------------------------------------------
Jan Olav Mossik jomossik@XXX.XXX
http://home.sol.no/~jomossik/ukindex.htm
http://home.sol.no/~jomossik/flyalbum.htm
http://home.sol.no/~jomossik/seilalbum.htm
---------------------------------------------------------
```
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