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:

Los Roques 2026


It's winter but not in the Caribbean, so welcome to Venezuela's Islas Los Roques archipelago for a 20nm sprint through these beautiful islands. Take care with your mutinous crew, they may be tempted to BBQ and snorkel!
Race #2007
INFO by brainaid.de
SOTO 30 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
RANKING:
SPQ1 - SPCH - SYC
RACE CLOSE: Wednesday,
February 11 at 2300 UTC.
Race starts: Feb 07th 13:00 Registration Open!
▶ Flash
GO TO RACE

Cross the Convergence 2026 - Samoa to Hawaii


The second race of our new series of ocean voyages across the World’s convergence zones, takes us northward again up the Pacific Ocean, now from Samoa at 13.5 degrees South, across the Equator to Hawaii, famed for its breaking surf and active volcanoes, on the edge of the Tropic of Cancer at 19.5 degrees North. It’s 2300nm, so we’ll take our very steady Steinlager II. Race #1982
INFOby brainaid.de
Steinlager IIPARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: CCZ - SYC
Race starts: Feb 06th 18:00 Registration Open!
▶ Flash
GO TO RACE

Beketov by Balloon 2026 - In Siberia


Way-back-when France and England, and Spain as well, were squabbling about who should control what of North America, only to lose the most of it to their own insurging colonists, their eastern European neighbour empire, Russia, was quietly assimilating vast, thinly nomadically populated territory of its own – Siberia! Amongst the many explorers and adventurers that served Russia so well in these conquests was the Cossack hetman Pyotr Beketov. Let’s pretend we’re he and eschewing horseback, let’s travel by balloon from Yekaterinberg to Krasnoyarsk – just a short 2000km (1100nm) section of the Trans-Siberian Express’ 9289km!
Race #1972
INFOby brainaid.de
SOL Balloon PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: BOL - SYC
Race starts: Feb 03rd 13:00 Registration Open!
▶ Flash
GO TO RACE

West Papua to Oregon 2026

Our next race in our Ocean Championships is the first of a series of great natural migrations we will track in 2026. The leatherback turtle is the most widely distributed marine reptile on planet Earth, and chooses to breed in warm tropical waters, but prefers to forage in more temperate habitats, travelling thousands and thousands of miles effortlessly annually to maintain this way of life. Our race will follow one of the typical trips of this turtle, from breeding grounds in the seas off West Papua to the coast of Oregon. It’s only 5900nm, so to keep up, but in comfort, we’ll follow in our Gunboat 90.
Race #2006
INFO by brainaid.de
GB 90 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ1 - OCCH - MIG - SYC
Race starts: Feb 02nd 11:00 Registration Open!
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member KaSToR
  2. Sailonline Yacht Club Member WRmirekd
  3. Sailonline Yacht Club Member CriticalHippo
  4. Sailonline Yacht Club Member rafa
  5. Sailonline Yacht Club Member rumskib
  6. Sailonline Yacht Club Member Sax747
  7. Sailonline Yacht Club Member vida
  8. Sailonline Yacht Club Member bonknhoot
  9. Sailonline Yacht Club Member Kipper1258
  10. Sailonline Yacht Club Member Panpyc

View full list

Series

Mobile Client

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

The mobile client