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
346
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
346
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:

ORCV Melbourne to Hobart 2024


Welcome to the legendary "Westcoaster," a race that has tested sailors for over 50 years. Spanning 435 nautical miles, this iconic journey takes racers from Melbourne, across the formidable Bass Strait, and around Tasmania's rugged west coast to the finish line in Hobart. Organized by the Ocean Racing Club of Victoria, the Westcoaster is renowned for its challenging conditions and adventurous spirit. This year, our SOLers will tackle the course in the swift and capable First 47.7, a yacht launched by Beneteau in 2000, built for speed and endurance in long ocean races. Get ready for an exhilarating ride in one of Australia’s most celebrated yacht races!
Race #1841
INFO by brainaid.de
First 47.7 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC
Race starts: Dec 26th 23:00 Registration will open soon
▶ Flash
GO TO RACE

Refeno Rally 2024


Although since 1986 this 300nm course from Recife, Brazil, to Fernando Noronha Island has been run many times IRL, we have only used this route twice in SOL. This time it will be the route of the last race in the ARCH 2024 classification.
Although the winner of the general classification can already be determined, the fight for the next places on the podium should be fierce. There are several contenders and Dofour34 have them at their disposal. Since we are starting in Vida_Maldita's home waters, he will give us a 30-minute handicap. Or maybe we should give it to him? 😊
Race #1865
INFO from brainaid.de
Dufour 34 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
ARQ4 - ARCH - SUPSOL - SYC
Race starts: Dec 23rd 17:00 Registration Open!
▶ Flash
GO TO RACE

Tristan da Cunha TIMED Race 2024

Originally designed by NZL_Scotsman, we head for the remote outpost of Tristan da Cunha where gales and storms may come and go but the sea is everlasting. This 64nm TIMED race has been raced on several occasions before, and this time we will be doing it in Rustler 36s.
RE-REGISTER HERE to race again after finishing a run
Race #1875
INFO by brainaid.de
Rustler 36 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ4 - TRCH - SUPSOL - SYC
REGISTRATION CLOSE: Saturday, 28 December at 22:00 UTC
RACE CLOSE: Saturday, 28 December at 23:00 UTC
Race starts: Dec 15th 12:00 Registration Open!
▶ Flash
GO TO RACE

The Beagle in the Azores 2024


And west the Beagle went one final time to round the Cape of Good Hope on May 31 1836. Ashore, Darwin met up with the Astronomer Royal, Herschel, who just happened to be in South Africa at the same time mapping the southern skies. After sounding out his many ideas on evolution and extinction with Herschel, the Beagle now sailed north, stopping briefly in St Helena, Ascension and Brazil, and the Cape Verdes again, to reach the Azores on September 19, 1836. Fitzroy and Darwin at this stage were in a hurry to get back to Britain, but online in 2024 we will stay a little longer to explore this final remote archipelago in our virtual Tall Ship Class B Beagle, 630nm round and about the islands, minding as always in these waters not to bump into any large cetaceans!
Race #1758
INFOby brainaid.de
Tall Ship Class B PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SVF - SYC
RACE CLOSE: Friday,
December 27 at 2300 UTC.
Race starts: Dec 14th 13:00 Registration Closed
▶ Flash
GO TO RACE

Christmas(W) to Christmas(E) Island 2024

Two waypoints, start and finish, and between them 5827 NM of two oceans. Not much time if we want to sit at the Christmas Eve table. You will need to maintain an average speed of over 10 knots, so prepare your 90ft well and choose the best of the many possible routes. Hands up everyone who knew there were TWO Christmas Islands?! OK, maybe you have sailed this Sailonline course before... but it's time to get ready for the 2024 challenge of a reversed race, starting in the Indian Ocean, racing to the Christmas Island in the Pacific! It also marks the conclusion of our prestigious Ocean Race Championship 2024. Please have fun! Fair winds!
PRIZE: SMPF
Race #1868
INFO by brainaid.de
90ft Monohull PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ4 - OCCH - SUPSOL - SYC
Race starts: Dec 02nd 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 FreyjaUSA
  3. Sailonline Yacht Club Member Vida_Maldita
  4. Sailonline Yacht Club Member CriticalHippo
  5. Sailonline Yacht Club Member rafa
  6. Sailonline Yacht Club Member Kipper1258
  7. Sailonline Yacht Club Member Satori
  8. Sailonline Yacht Club Member Sax747
  9. Sailonline Yacht Club Member BRENTGRAY
  10. Sailonline Yacht Club Member CollegeFund

View full list

Series

Mobile Client

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

The mobile client