Sailonline.org - Online Sailing

Facebook

Login

Support Sailonline

If you haven't already - join the SAILONLINE YACHT CLUB!

Please also consider making a donation - all amounts are greatly appreciated!

Tweets by @sailonline

Board » General Discussion » DTF calculation

Posted by NZL_Scotsman Posts 251 Registered at 2008-10-04 07:59:40	2012-05-01 09:57:51 - DTF calculation
	One of the most frequently asked questions on chat is how the distance to finish is calculated. I've tried to explain that, based on my understanding. If anyone knows differently, please add to the thread. The first two plots here are sketches of a race similar to our SWR Leg 6. That is a good example of where the DTF is not entirely intuitive. The third plot is the mark to mark distance, which many think should be the DTF. The distance to the next mark is determined by the closest distance to the line that goes out from the mark. This line (the mark 2 rounding line) is drawn at right angles to the line between marks 1 and 3, starting at mark 2 and continuing infinitely. From that point, the next part of the DTF is the distance to the next mark. Then mark to mark to the finish. So looking at the attached plots. Boat A sails close to the rhumbline, so its DTF is very similar to a mark to mark course. Boat B, however, is sailing wide of the mark. From B1 we find the closest point on the rounding line (point X). From there to mark 3. All the distances from the various positions are shown on the plots. --- Last Edited by NZL Scotsman at 2012-05-01 10:11:12 --- Attachments 1-existing.JPG (199.6 KB) 2-existing.JPG (260.3 KB) 3-mark2mark.JPG (191.4 KB)
Posted by NZL_Scotsman Posts 251 Registered at 2008-10-04 07:59:40	2012-05-01 10:15:28 - Re: DTF calculation
	If you have another look at the mark to mark system, the main problem with this is that when you approach the mark rounding line wide - then the DTF includes the distance to this mark until you pass it. Then it jumps to the next mark. For example Boat B at B4 jumps from 43nm to 35nm as soon as the line is crossed. The existing system has a similar problem, but it is much less severe. The problem only really occurs when you have a situation like B3, where the mark 3 rounding is very wide and therefore the mark 3 to 4 distance is much greater than compared to boat A. (35nm compared to 17nm).
Posted by NZL_Scotsman Posts 251 Registered at 2008-10-04 07:59:40	2012-05-01 10:25:24 - Re: DTF calculation
	Here is an alternative method of calculating DTF that might avoid some of the issues the current version creates - those rare occasions when the course shape results in longer DTF than is reasonable. The attached plot will help with this description. Instead of waiting until the mark is rounded before the next mark is considered, DTF is determined by going to the next mark as soon as the extended bearing is crossed. If the boat crosses back, then simply revert to distance to the mark. For example B1 has crossed the line, therefore DTF is distance to Mark 3 plus 3-4 distance (141+17=158nm). This compares to 166nm with current method. Now compare B2 and B3. The boat has only moved about 15nm. DTF for B2 = 108nm, B3 = 98nm. Sounds reasonable? The existing method B2 - 112, B3 - 111. I think this method will be easy to compute, so it will not add to the server requirements. I cannot think of any disadvantages, but might be worth testing it with some more examples. Attachments 4-alternative_1.JPG (195.5 KB)
Posted by AGage Posts 411 Registered at 2008-12-27 10:31:57	2012-05-02 06:36:49 - Re: DTF calculation
	My personal solution is to ignore ranking and sail, all will be sweet when we cross the line Your method of calculating is slightly off. Example 1 shows best (at our current situation on VOR6) is how it is currently done, but rather than orthogonal to the line 1-3, the 'scoring line' that extends from 2 to score roundings bisects the angle 1-2-3. OK, in your diagrams this does not make a lot of difference - but shift 2 along the direction of line 1-*3 and you can see it can... This is the BRG value on the NOR - Brg from ship to mark at the instant the server scores your rounding. Just some advice from experience (regarding alternative methods) - you can draw 100 examples and it all works fine, but there is often some edge case where things go nuts... Diagram 4 does appear a better method of computing DTF, but not of scoring roundings (B3 should not be considered to be on Leg3 for example, and could flip from Leg2 to 3 many times on its way to the finish? sim. B1) and we still need the line we currently have extending from the mark to score roundings (and to detect 'unwindings') This would be where is creates more load to compute - 2 lines crossings per mark instead of 1 - actually 1 line crossing (score) and 1 'which side of the line are we' (dtf) (not saying is a bad idea, just commenting because you did about server loads) It's great that others are thinking on things like this though - when you are doing such examples though, try pulling out GE and drawing LONG courses, as these are really the only ones where the ranking ever becomes an issue for some - when a GC path significantly changes where on the line the scoring point is, compared to a Loxo (and also the scoring line itself, being GC) - small distances like in your diagrams don't show this effect. Also GC can change the edge cases a bit - remembering back to the ranking fiasco on Leg 5 while the scoring point was the south pole for pretty much every boat. Of course, the other question is "Is this a good allocation of development resources?" - which always needs to be asked I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by Amigo_do_Boteco1 Posts 4 Registered at 2008-01-18 11:07:00	2012-05-04 01:14:52 - Re: DTF calculation
	Thank you NZL Scotsman This is a very complicate way to measure the DTF. I think its not that hard to implement a way to calculate the DTF based on a point to point measure: - point to point for "mark to mark", and - point to point for "boat to next mark". Its a suggestion to SOL programers. Fair winds --- Last Edited by Armando Faria at 2012-05-04 02:33:41 --- Boteco1 - Make Bore Impossible
Posted by AGage Posts 411 Registered at 2008-12-27 10:31:57	2012-05-11 06:49:06 - Re: DTF calculation
	It is point to point for future legs?? It is actually point to point for all legs, just for the first 2 the exact location of that point is not fixed... It isnt really any more complicated than just computing boat to mark to mark etc, but it is more realistic. It is exceedingly difficult to write an algorithm that can most accurately handle all possible course configurations, sizes, and locations (for example, had the VOR Leg5 been translated N so that it crossed the Equator, but otherwise remained the same shape, we would not have had the ranking issues with the South Pole) If we change it then boats that stay close to the rhumbline will most likely figure higher in the rankings than they are. Personally I would rather appear lower in ranking than I really am. There is no realistic way to have a consistently accurate ranking across the whole race - given the very different configs/sizes/locations of SOL races and the variety of possible routes. Rank calculations and visualisations like you see on the AC45s and VOR inport races only work for pure Windward/Leeward courses, and even then as the wind becomes more shifty they become less accurate. I don't really see there is much wrong with the current system I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__

Please login to post a reply.

Races

Next Race: 00d 00h 00m

Current Races:

Kamchatsky to Tromsoe 2024

Just before the light failed entirely in Antarctica, Skip Novak managed to get his Pelagic 77 out of the ice and north up the Pacific to Kamchatsky. The sun is shining (24/7) and not a penguin to be seen, and Skip believes the Norh East Passage is ice-free (sufficiently) to attempt it, so all aboard for a c 4250nm voyage from east to west across the top of Siberia, Russia and Norway to Tromsoe!
Race #1779
INFOby brainaid.de
Pelagic 77 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: HLAT - SYC
Race starts: Jul 18th 19:00 Registration Open!

▶ Flash

GO TO RACE

Cowes to St Malo TIMED Race 2024

Organized by the Royal Ocean Racing Club since 1929, the Cowes to St Malo Race is a true RORC Classic. Starting from the Royal Yacht Squadron Line, Cowes, IOW, a magnificent spectacle can be watched from The Parade, Cowes. The Cowes to St Malo Race is part of the RORC Season’s Points Championship, the world’s largest offshore racing series. Dating back to 1906, the Cowes to St Malo Race precedes all of the world’s famous races including the Fastnet Race. This is a TIMEDrace, so you may RE-REGISTER HERE to try again, after finishing a run. You will have 13 days and 11 hours to show your skill and decision making after the race opens.
Race #1828
INFOby brainaid.de
TP-52 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ3 - TRCH - SUPSOL - SYC
RACE CLOSE: Saturday,
27 July at 23:00 UTC
Race starts: Jul 14th 12:00 Registration Open!

▶ Flash

GO TO RACE

Lake Ontario 300 Challenge 2024

Lake Ontario Offshore Racing (LOOR) welcomes Sailonline to a second race of their annual series, this time as hosts of the virtual LO300. As the name implies, the LO300 is a 300nm offshore race across the length and breadth of Lake Ontario, from and back to Port Credit YC. Once again, as is our wont on Lake Ontario, a fleet of Beneteau First 36.7s is being made available to virtual racers. There is an overall prize for the SOLer who best bosses Lake Ontario over the two races hosted on SOL, this 300 and the Susan Hood raced back in May; so let the competition be fierce!
Race #1805
INFOby brainaid.de
First 36.7 PARTICULARS
NAM_AWIP WX Updates:
0245 / 0845 / 1445 / 2045
Ranking: LOOR - SYC
Race starts: Jul 13th 15:10 Registration Open!

▶ Flash

GO TO RACE

SSANZ Triple Series 2024 - Race 1

Welcome once again to our online buddied Short-handed Sailing Association of New Zealand brilliant long-standing SSANZ Triple Series for two-handed yachts on the waters of the Hauraki Gulf, sponsored this year by Lewmar Marine. Commencing with a c 46nm quick dash out to Motuora Island via The Haystack and home, we will as always compete in virtual Young88s, against the real-life fleet of the members of the ever-Young 88 Association!!
Race #1823
INFO by brainaid.de
Young 88 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SSANZ - SYC
Race starts: Jul 12th 21:15 Registration Open!

▶ Flash

GO TO RACE

Tall Ships Races 2024 - Tallinn to Turku

Welcome to the second of three virtual Tall Ships Races on the Baltic Sea which are concurrently being organized in-real-life Sail Training International . This second race is from Tallinn, Estonia to Turku, Finland; circa 150nm in Sailonline’s stately Full Rigger.
NOTE: Starts and Finishes in tall ships racing are always offshore to avoid conflict with shipping and shipping lanes.
Race #1812
INFO by brainaid.de
Full Rigger PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: TS - SYC
RACE CLOSE: Friday,
July 19 at 2300 UTC.
Race starts: Jul 11th 14:00 Registration Closed

▶ Flash

GO TO RACE

Southampton to Punta del Este 2024

Sailonline is delighted to offer our sailors a 'reversed' Atlantic ocean race. As the RTW championship Atlantic part takes a detour to the Bahamas, we will offer you a reversed course as we will not do the 'usual' leg this year either. It is the July edition of this year's Ocean Championship. Our boat is the OD_65v3.
PRIZE: SMPF
Race# 1820
INFO from brainaid.de
OD65 PARTICULARS
WX updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ3 - OCCH - SUPSOL - SYC
Race starts: Jul 01st 11:00 Registration Closed

▶ Flash

GO TO RACE

Go to race archive

SYC Ranking

View full list

Series

Mobile Client

SYC members have the benefit of access to our mobile/lightweight web client!

The mobile client