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 » General Discussion » SOL maps Datum & Interpolations.

Posted by
JB


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
2015-04-16 16:47:25 - SOL maps Datum & Interpolations.
Viva all,

Two questions for the STC:

1 - What is the SOL maps Datum?
2 - What is/are the algorithm/s used for (TWS; TWD) interpolation?

Thank you STC.
Sail Fair.
Posted by
A2


Users avatar

Posts
347
Registered at
2009-10-26 00:39:42
2015-04-16 17:20:59 - Re: SOL maps Datum & Interpolations.
SOL uses data from GSHHG (Global Self-consistent, Hierarchical, High-resolution Geography Database)

http://en.wikipedia.org/wiki/GSHHG

Data for each resolution is formatted to tiles which are visible in the game. The tiles are compressed files with XML-formatted polygons (list of lat/lon coordinates) of the shorelines of islands and continents.
None so blind
Posted by
kroppyer


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
2015-04-16 18:22:58 - Re: SOL maps Datum & Interpolations.
sol uses h and i resolutions of GSHHG, and c and l resolutions are used in the client only for when you are zoomed out and don't need to see all details.

Interpolation:
this is what I found by measuring and fitting:
u/v interpolation (not TWS/TWD)
over space: linear
over time: I found this a close match (but not exact):
previous+(next-previous)*(0.5+0.5*cos(t/(2pi)))
with 0<=t<=1, previous the value for t=0 and next the value for t=1.

jakob said @ 2010-12-03 16:17:40:
Hi

So, about the interpolation of weather in time and space :-) Maybe some out there is longing for a short one-liner as explanation but I am afraid there is a little more to it… Here is in short the steps we take to get an "observation" (as we call it) at an arbitrary time (called t) and location based on forecasts.

1) Find the two forecast time steps (here called t1 and t2) surrounding the time t.
2) In space we simply use linear interpolation (in the 2 dimensions lat/long) of the forecast-data in the 4 closest forecast-nodes in the grib-file. Since the forecast has separate fields, one for northerly wind and one for easily wind we need to treat these entities separately, in our case by using complex notation. We also need to do this for both frames t1 and t2 from step 1 which thus gives us a set of two complex-valued space-interpolated wind speeds, one at t1 and one at t2.
3) Now we need to interpolate in time as smoothly as we can. We chose to do this using simple shape functions ensuring continuous wind speed and actually in our case continuous time derivatives.

Now, there are naturally many ways of doing this. One draw-back of this kind of interpolation is that we loose energy. Imagine the situation with constant wind speed but rotating wind direction so that at t1 the wind is due easily and at t2 it is due westly. In our interpolation we would (falsely) get that the wind speed at the time (t1+t2)/2 would be zero.

Also note that we do not always use the GFS model from NOAA. In particular around Sweden we have more detailed forecasts.

Hope this helped a little.

Regards
Jakob


(end quote)
Posted by
JB


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
2015-04-17 11:04:45 - Re: SOL maps Datum & Interpolations.
Viva “A2”, Huib and Jakob,

Thank you for your quick and detailed replies.
Following now more comments.
Sail Fair.
Posted by
JB


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
2015-04-17 11:05:14 - Re: SOL maps Datum & Interpolations.
Viva Huib

From what I understood the Wind vector field is divided in two vector components along orthogonal directions: North -South (“u” Wind vector component) and East - West (“v” Wind vector component) - see also included file.

Confirm pls if your approximation formula for interpolation over time (between time “t0” and time “t1”, with t1 > t0) is:

u1 = u1 + ( u1 - u0 ) x ( 1/2 + cos ( t/2PI )/2 ).
v1 = v1 + ( v1 - v0 ) x ( 1/2 + cos ( t/2PI )/2 ).

Did you managed to find a sigma (STD) for your (time, distance) interpolation results?

Thank you Huib.
Sail Fair.
Posted by
JB


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
2015-04-17 11:06:14 - Re: SOL maps Datum & Interpolations.
Viva Jakob,

Very interesting this subject, to say the least.
I’m one of the SOLers being puzzled for a long time, first, with the process for the SOL Wind interpolation process. “How it’s done?”
Secondly, the ruthless lack of consistency between routing software results and GRIB data in the low wind range, i.e., aprox. 0 < TWS < 6,00 - 8,00 Kts, independently of the GRIB given TWA, normally obtaining over rated BS.

For the last issue I strongly believe that that are two main causes for this discrepancies:

1 - Different Wind interpolation processes (routing software vs. SOL one);
2 - Time step calculation used on routing,

IMHO the second factor is the preponderant one.
If we think that, for instance, the minimum step calculation for qTVlm is 5 minutes, and if we have an 8º/minute induced Wind veering, difficulty you reach a 5 minute distance in the routing software with the calculated BS, not to say TWA. The implications are obvious.
But now, it’s time for understanding the first factor: Wind interpolations.

So, for the”distance” interpolation SOL uses bi-linear interpolation.
For the “time” one you mentioned “simple shape functions”, Can you pls detail more what are those “shape functions (linear, polynomial, 3D mesh, …)?

Thank you Jakob.


--- Last Edited by JB at 2015-04-17 11:09:16 ---
Sail Fair.
Attachments
Posted by
kroppyer


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
2015-04-17 22:47:27 - Re: SOL maps Datum & Interpolations.
I quoted Jakob from a post from 2010, I don't expect him to elaborate on that soon.

The formulas show you understood what I meant. Use t=t'/(t1-t0)-t0 to "normalise" t to [0,1]. I have no real sigma, but from what I remember it's around 2% at t=0.25/t=0.75, it's exact at t=0,0.5,1
It's weird to use a cos for this purpose, it's relatively heavy to calculate and far to precise for something that doesn't need geometry. Sol probably uses some 3rd to 5th order polynomial approximation that is faster to calculate, but slightly different from cos().

Please login to post a reply.

Races

Next Race: 00d 00h 00m

Current Races:

Tobruk to Iskenderun 2026

Welcome to the fourth of our nine races across the length and breadth of the Magical Mediterranean Sea, and the second that is not in partnership with a real-life event, this time for good reason, which we won’t get into here. From Tobruk to Iskenderun it’s 670nm in length, with the added complication that the island of Cyprus will be in your way. Hoping, possibly forlornly, for plenty of downwind sailing, we’ll take the Lagoon 55 for the voyage!
Race #1976
INFO by brainaid.de
Lagoon 55 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: MED - SYC
Race starts: May 05th 15:00 Registration will open soon

▶ Flash
GO TO RACE

Tasmania to Kamchatka 2026

Welcome to yet another all-new route. Tasmania to Kamchatka, part of our Ocean Championship and also the third leg of the ROW (Round the Old World) series. Be aware of the obstacles on the way, likely to split the fleet. It's like X:mas to X:mas, crossing the same waters, navigating the same archipelagos. This second race of OCQ2 2026 will be sailed in our revised 'version 2' Imoca 60 Foil. Race #2031
INFO by brainaid.de
Imoca 60 Foil v2 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ2 - ROW - OCCH - SYC
Race starts: May 04th 11:00 Registration Open!
▶ Flash
GO TO RACE

Bay to Bay 2026 - Leg 2

After a thrilling opener, the 2026 edition of the Bay to Bay Yacht Race continues with Leg 2, bringing even more adrenaline and tactical excitement to the waters of the Great Sandy Marine Park. Our SOLers will once again take the helm of their ILCA 7, this time tackling a 24-nautical-mile course through shifting breezes and tight competition.
With fresh legs, sharper tactics, and everything to race for, it’s anyone’s game! See you on the start line for the grand finale!
Race #2024
INFO from brainaid.de
ILCA 7 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
RED - B2B - SUPBUD - SYC
Race starts: May 02nd 21:30 Registration Open!
▶ Flash
GO TO RACE

Bay to Bay 2026 - Leg 1

Prepare for an electrifying start to the 2026 Bay to Bay Yacht Race! This first leg will see our SOLers take on a fast-paced, 16-nautical-mile sprint through the stunning waters of the Great Sandy Marine Park, weaving past the breathtaking landscapes of the Great Sandy National Park. Racing in ILCA 7, the fleet will battle shifting winds, tricky tactics, and fierce competition—all in pursuit of victory! With the thrill of the race and an unforgettable sailing experience ahead, we can’t wait to see you on the start line!
Race #2023
INFO from brainaid.de
ILCA 7 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
SPQ2-SPCH-B2B-SUPBUD-SYC
Race starts: May 02nd 02:00 Registration Open!
▶ Flash
GO TO RACE

Regata dell’Accademia Navale 2026

Welcome to Sailonline’s partnership with the City of Livorno (Leghorn say the English) in cooperation with the local yacht clubs and the Italian Navy, offering all the online opportunity to race the famous Regata dell’Accademia Navale - 630nm in length. Starting and finishing in Livorno, this year the race takes you the length and breadth of the Tyrrhenian Sea via little Mortorio (Mortoriotto) off Sardinia to Capri and home again to Livorno. This race also counts towards Sailonline’s Magical Mediterranean Series and the SuperBuddy title. Online, you will be sailing a Mini 6.50, to give you a match against several of the real-life entrants, which you will be able to track on-screen!
Race #2020
INFO by brainaid.de
Mini 6.50 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: MED - SUPBUD - SYC
Race starts: Apr 25th 10:00 Registration Open!

▶ Flash
GO TO RACE

Guess Gotland TIMED Race 2026

The Guess about Gotland is do we helm our First 27 solo east or west of the island in this 140 nm race. This is a TIMEDrace so you may RE-REGISTER HEREto try again after finishing a run. You will have 13 days and 11 hours to test your skill and decision making after the race opens.
Race #2030
INFOby brainaid.de
First 27 solo Particulars
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ2 - TRCH - SYC
RACE CLOSE: Saturday,
2 May at 23:00 UTC
Race starts: Apr 19th 12:00 Registration Open!
▶ Flash
GO TO RACE

Rommel by Balloon 2026 - Through the Sahara


Generalfeldmarschall Johannes Erwin Eugen Rommel may have been that chivalrous, gallant tank commander of legend, led astray by the Prussian military traditions of loyalty and chain of command, but one thing he wasn’t, was mad. So, let’s do it anyway – not stick to the coast and the battle for logistical superiority – but depart the oases of Siwa south west into the desert for Agadez and its uranium. And let’s go by balloon, it’s only 2500km (1250nm)!
Race #1973
INFOby brainaid.de
SOL Balloon PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: BOL - SYC
RACE CLOSE: Sunday,
May 3 at 2300 UTC.
Race starts: Apr 07th 13:00 Registration Closed
▶ Flash
GO TO RACE

Falklands to Tasmania 2026

Welcome to yet another all-new route. This next race is part of our Ocean Championship as well as the ROW series. Falklands to Tasmania, is like last year's island to island. It’s one of the longer ones 6400nm, more than half way around the world (and the Antarctica). It starts the 2026 OCQ2, and will be sailed in our Steinlager 2, contrary to prior notice.
Race #2021
INFO by brainaid.de
Steinlager 2 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ2 - ROW - OCCH - SYC
RACE CLOSE: Thursday,
May 7 at 2300 UTC.
Race starts: Apr 06th 11:00 Registration Closed
▶ 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 rumskib
  5. Sailonline Yacht Club Member rafa
  6. Sailonline Yacht Club Member Panpyc
  7. Sailonline Yacht Club Member CollegeFund
  8. Sailonline Yacht Club Member sassy63
  9. Sailonline Yacht Club Member vida
  10. Sailonline Yacht Club Member Sax747

View full list

Series

Mobile Client

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

The mobile client