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Board » Technical Support » * * * Vic-Maui Malfunction Alert * * *

Posted by
A2


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2009-10-26 00:39:42
2010-06-26 01:46:47 - * * * Vic-Maui Malfunction Alert * * *
Passing the Cape Flattery Waypoint to the North when "Rounded" causes the Boat Ranking to place you at the bottom of the list with a DTG of 2704.1 nm in a race that is 2311.4 nm long.
None so blind
Posted by
Eddie C. Dost


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2008-08-23 11:40:29
2010-06-26 11:01:07 - Re: * * * Vic-Maui Malfunction Alert * * *
This is a small problem with the way SOL calculates the distance to go:

You need to look at the half angle line from the next waypoint to pass. SOL searches for a great circle passing this line orthogonally and going through the position of your boat. In this case the great circle passes the waypoint Nakalele at some distance, so you get a longer distance to the waypoint and a longer distance from there to the finish.

See attached sketch for details.

The above algorithm is necessary so the DTG function is continuous and there are no jumps in the ranking during passing of a waypoint.

The real bug is in the overall calculation of DTG, the above algorithm is only applied to the next waypoint to pass, if there are more waypoints the distance of the great circle going through these waypoints is used in the calculation. This causes the jump in DTG after passing Cape Flattery.

As the ranking issue will sort itself out once the boats are about 400nm offshore, I'd consider this a minor problem, rankings will be fine for the remaining 2000nm of the race.

_/)_/)_/)
The sea is lovely, dark, and deep, But I have promises to keep, And miles to sail before I sleep, And miles to sail before I sleep.
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Posted by
RainbowChaser


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2009-06-14 09:44:50
2010-06-26 11:22:22 - Re: * * * Vic-Maui Malfunction Alert * * *
Super explanation... even my little grey cells comprehend!!

Thank you :-)
Posted by
A2


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2010-06-26 11:41:36 - Re: * * * Vic-Maui Malfunction Alert * * *
Thanks for the technical details of the situation. I had not encountered this sort of behavior in any previous races.

P.S,

Thanks for the graphic & expanded explanation,

Standing down from * Alert * status :)

--- Last Edited by Aethereal at 2010-06-26 12:19:06 ---
None so blind
Posted by
Eddie C. Dost


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2010-06-26 11:54:16 - Re: * * * Vic-Maui Malfunction Alert * * *
The half angle line is derived as follows:

Take the line (I think it's the loxodrome) from the previous waypoint (or start point) to the waypoint in question and the line from the waypoint in question to the next waypoint (or finish). These two lines of the race track define two angles, a greater "outside" angle and a smaller "inside" angle. The "outside" angle is on that side where the waypoints needs to be passed. The line cuting this angle in two halfs is the "half angle line" I was referring to.

When your boat passes this line, the server will mark the waypoint as passed (if passed in the correct direction, of course).
_/)_/)_/)
The sea is lovely, dark, and deep, But I have promises to keep, And miles to sail before I sleep, And miles to sail before I sleep.
Posted by
nacrr


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2010-01-01 15:57:17
2010-11-10 19:34:20 - Re: * * * Vic-Maui Malfunction Alert * * *
This "small" problem creates a huge mistake in the case of the race "Ron Cazar". Please find a way to solve the problem!
nacrr
Posted by
A2


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2010-11-14 17:06:54 - Suggestion for DTG Anomaly
Thanks to Eddie's technical description & some subsequent discussions with 76T an idea occurred to me on how to improve the DTG measurement without radically altering the method by which it is derived.

I post it here so that my understanding of how things actually work can be evaluated.

As it stands now the "Bearing to Mark" splits the difference (Half-Angle) between the incoming Rhumbline and its exit. Crossing this imaginary line is where a rounding is registered and the closest approach to this same imaginary line is how the DTG measurement is calculated.

Now if this "Bearing to Mark" was rotated forward in favor of the exiting Rhumbline until it was perpendicular to it, then the closest approach measurement would more closely resemble the actual course distance.

My suggestion is to give the Bearing to Mark a forward bias.
None so blind

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