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!

Board » Technical Discussion » How to handle wheatherdata differences ?

Posted by
Armchairsailor


Users avatar

Posts
6
Registered at
2012-03-22 09:35:22
2015-09-29 02:43:55 - How to handle wheatherdata differences ?
In my current position in the Timed transat
LAT 49°14.3747'LON W 022°55.8096'
DATE 2015-09-29 TIME 02:30:08
the wind information in the client
differs from the wind information in the gribviewer, which uses the file from
brainaids site:
"weather_186_global_gfs_20150928_2227.grb"

client TWD 122.34° TWS 2.82KT
gribview TWD 120 TWS 5.3KT

Meanwhile I have read that there are differences. The effect is that my routing software sends me in the wrong direction.
How do other SOL users handle this situation ?

--- Last Edited by Armchairsailor at 2015-09-29 08:17:01 ---
Posted by
kroppyer


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
2015-09-29 17:42:44 - Re: How to handle wheatherdata differences ?
The difference is there because sol uses a different interpolation method than your router/grib viewer does. Some routers/viewers offer multiple interpolation methods. If your viewer/router has that option, you can experiment with the different methods, see which one is closer.

In general, this problem occurs most often in large wind shifts (spacial, or over time). So usually it's in the light winds ("blue goo") that the router is off. I believe most people cope with that by hand steering as much as possible in the blue goo, and even better: avoiding the blue goo.

You could consider it as part of realism: winds are not always as they are in your router, especially not in light, shifty conditions.

If you're really bothered by this, you could try different routers/viewers to see which matches sol the closest, and you could write your own/adapt an existing one.
Posted by
Armchairsailor


Users avatar

Posts
6
Registered at
2012-03-22 09:35:22
2015-09-30 04:31:59 - Re: How to handle wheatherdata differences ? thnks
Tnks kroppyer for your reaction.
It is as you say. This resembles reality.
I was attrackted by the high wind speeds of the depression, but development of it changed, as usual. I had no chance at all to avoid the blue gue afterwards.
I am having thoughts to write my own router. But that won't change reality.
Posted by
Kipper1258


Users avatar

Posts
251
Registered at
2015-06-18 22:08:49
2015-10-12 22:23:32 - Re: How to handle wheatherdata differences ?
kroppyer said @ 2015-09-29 17:42:44:
...you could write your own/adapt an existing one.
Kroppyer, where can we source the SOL server interpolation code to see how it works / replicate it in other tools? It does not seem to be posted anywhere (at least Google can't find it)

Thanks
Posted by
kroppyer


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
2015-10-13 13:35:49 - Re: How to handle wheatherdata differences ?
Server code is not published, all we (solers) can do is measuring and fitting. I've done this in the past, didn't get perfect results but to me it's close enough for now.

In this thread I explain my findings, and I link to a post from Jakob (who, together with kalle, wrote most of the server-side code) explaining some more about the interpolation.

EDIT: fixed link (you should'be said it didn't work! :)

--- Last Edited by kroppyer at 2015-10-20 14:23:32 ---
Posted by
outlaw


Users avatar

Posts
33
Registered at
2012-06-01 13:37:55
2015-10-13 18:19:01 - Re: How to handle wheatherdata differences ?
Interpolation in space is simple bilinear. In time is a 'shape function', apparently 3*t^2-2*t^3. So you get
v(19:00)=.26*v(18:00)+.74*v(21:00)

For the polar use the 'detailed csv', where TWS interpolation is again linear. Not sure about TWA, but it's 1 degree resolution, *shrug*.
Posted by
kroppyer


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
2015-10-13 23:04:01 - Re: How to handle wheatherdata differences ?
For interpolation on the polar, I used the same method in SPINNACER as AGage used in his SOL Polar Explorer. See class AGage.PolarExplorer.Interp here.


AGage comments (in the code) "Interpolate as on sailonline.org", so I guess he did exactly the same as sol does.

I'm not sure if this code performs linear interpolation, but I wouldn't know what else. I'm pretty sure it's at least close to linear interpolation. Also "linear" looses meaning when you use polar coords... but now we're talking minor differences. Like outlaw says: *shrug*

Please login to post a reply.

Races

Next Race: 00d 00h 00m

Current Races:

Newport Bermuda Race 2026


The 636-mile Newport Bermuda Race is one of the oldest regularly scheduled ocean races, and with the Fastnet and the Sydney Hobart Race, it is one of the three great classic races of the yachting world. First raced in 1906, now in its 54th biennial, the Race can again also be participated in online. In real life, the race is open to entry in four different division – Double-Handed, St. David's Lighthouse, Gibbs Hill Lighthouse, and Finisterre Division – which in the past we have also offered online. This year, however, our online race will be all-in one-design in silky swift Club Swan 50 yachts, so that your results can count towards SOL’s SUPerBUDdy title, whilst hopefully still giving you a chance of overall line honours against the fastest yachts of the real-life Gibbs Hill Division, especially if you can find some forecast winds that don’t quite work out that way on the real water!
Race #2038
INFOby brainaid.de
Club Swan 50 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SUPBUD - SYC
Race starts: Jun 19th 17:05 Registration Open!
▶ Flash
GO TO RACE

Shetland Race 2026 - Leg 1


Get ready to tackle the North Sea as we sail from Norway’s stunning outer Korsfjorden to the rugged Shetland Isles! Welcome to an exciting new chapter for this classic 190-nautical-mile ocean route, bringing our global community together for a true test of strategy and seamanship.

This year, the race is a proud stop in Sailonline's 2026 Super Buddy Championship, which features our real-life racing partnerships over the calendar year. We are stepping aboard the Linjett 33, a sturdy Swedish cruiser-racer designed by Mats Gustavsson and built by Rosättra Båtvarv. Seasoned veteran or eager newcomer, prepare for intense competition.

Plot your course and see you at the start line!
Race #2039
INFO by brainaid.de
Linjett 33 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SHE - RED - SUPBUD - SYC
Race starts: Jun 17th 11:00 Registration will open soon
▶ Flash
GO TO RACE

Bluefin Tuna - Mexico Gulf to North Sea 2026

Join us and follow the Bluefin Tuna, as they migrate from their spawning grounds in the Gulf of Mexico to their feeding grounds in the North Sea off the coast of Norway. This 4300 nm race is part of our Ocean Championship and the final race of OCQ2 2026, and also the second leg of our Migration series. We will be sailing in our Gunboat 90. Race #2035
INFO by brainaid.de
Gunboat 90 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ2 - MIG - OCCH - SYC
Race starts: Jun 08th 11:00 Registration Open!
▶ Flash
GO TO RACE

Thessaloniki to Port Said 2026

Welcome back to the Eastern Mediterranean for the fifth race of our Magical Mediterranean Series. Only marginally less unlikely to be ever raced in real life during SOL’s lifetime than our previous race from Tobruk to Iskenderun, this 710nm race takes us from Greece’s ancient northern port of Thessaloniki through the Aegean island maze, south east into the Levantine Sea to a finish in modern Port Said, in splendid Swan 65s!
Race #1977
INFO by brainaid.de
Swan 65 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: MED - SYC
RACE CLOSE: Wednesday,
June 17 at 2300 UTC.
Race starts: Jun 06th 15:00 Registration Closed

▶ Flash
GO TO RACE

TuziTekwini TIMED Race 2026

Sailonline is delighted to have been invited again to partner with the King Shaka Yacht Club (KSYC) of South Africa to bring a virtual version of the Tuzi Tekwini Ocean Raceto our online community.
The Tuzi Tekwini Ocean Race is a near 90 nautical mile race down the east coast of South Africa from the port of Richards Bay to the port of Durban. Achievable in a single day and seen by some as an easy mile-builder, the course is more tactical than some think and should not be underestimated. This iconic race promises to be an unforgettable experience for yachting enthusiasts.
Our online version of the race will be a TIMEDrace so you may RE-REGISTER HEREto try again after finishing a run. You will have two weeks to test your skill and decision making after the race opens.
Race #2037
INFOby brainaid.de
Sailonline’s virtual yacht for all your runs will be our much-loved Farr 38 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ2 - TRCH - SUPBUD - SYC
RACE CLOSE: Saturday,
13 June at 23:00 UTC
Race starts: May 31st 12:00 Registration Open!
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member WRmirekd
  2. Sailonline Yacht Club Member KaSToR
  3. Sailonline Yacht Club Member CriticalHippo
  4. Sailonline Yacht Club Member bonknhoot
  5. Sailonline Yacht Club Member rafa
  6. Sailonline Yacht Club Member Panpyc
  7. Sailonline Yacht Club Member HoratioPugwash
  8. Sailonline Yacht Club Member YANN
  9. Sailonline Yacht Club Member CollegeFund
  10. Sailonline Yacht Club Member sassy63

View full list

Series

Mobile Client

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

The mobile client