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few remarks:

Velocity & speed: as a vector physical quantity. The scalar absolute value of 
the velocity vector is speed, and direction is required to define it.
The relation between vessel's motion, and Sun motion is not arithmetic, is a 
composition of the two motions, a vector composition. The arithmetic you 
explained is a first approach.

Running fixes (or DR): to move one position to another, an inertial reference 
frame is needed, the ground is adopted usually. So the speed for all the 
calculations is the SOG, (Speed Over Ground) that is the vessel speed 
corrected by the effects of Current, Set & drift. Wind, Leeway, only change 
the direction of the velocity vector of the vessel.
In vector notation V over ground = V vessel + V current

Andr�s Ruiz
http://www.geocities.com/andresruizgonzalez


-----Mensaje original-----
De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de George Huxtable
Enviado el: mi�rcoles, 28 de mayo de 2008 16:31
Para: NavList@fer3.com
Asunto: [NavList 5173] Re: Navigation exercise

..., with due allowance made for any errors in ship's speed and 
course through the water and rate and direction of tidal set, stating the 
precision with which they need to be known.
...



-----Mensaje original-----
De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de frankreed@HistoricalAtlas.net
Enviado el: mi�rcoles, 28 de mayo de 2008 14:08
Para: NavList@fer3.com
Asunto: [NavList 5169] Re: Navigation exercise

...
The great part, George, is that you JUST DEMONSTRATED how easy it is to make 
the correction. Run your example in reverse: take the observed altitudes, 
SUBTRACT the effect of the vessel's motion, and then you have the altitudes 
that would be observed by a motionless observer. It is indeed an EASY 
process. We could teach a child to do it. The arithmetic and the concepts 
are that simple. 

You worried that we have to include not just the speed of the ship through the 
water (which is readily available in any modern vessel) but also currents and 
the changing declination of the Sun...
 
... And there will be an error if there are unknown currents: a four knot 
current after four hours would lead to a position error of 16 n.m. in a 
STANDARD running fix, if that current is not taken into account. This is a 
property of ALL running fixes, not a flaw with the particular method under 
discussion here. 
...

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