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:

Port Huron to Mackinac 2025


First run in 1925, the race from Huron to Mackinac is considered to be the world's longest consecutively run freshwater sailing race. Let's go - 260nm in X-362 Sport's awaits!!
Throw away unnecessary ballast from your boats. Fresh water has less buoyancy, which means greater draft and greater resistance to movement, which means lower speeds, etc.😊
Race #1959
INFO from brainaid.de
X-362 sport PARTICULARS
NAM_AWIP WX Updates:
0245 / 0845 / 1445 / 2045
Ranking:
ARQ4 - ARCH - SUPSOL - SYC
Race starts: Oct 20th 17:00 Registration Open!
▶ Flash
GO TO RACE

Sinbad by Balloon 2025 - Carnarvon to Dondra


From here, our home in Bharatavarṣa is now north west of us – said Sinbad to his fellow balloonists. Perhaps we can ride the wind first further north, and then catch the winds that every year bring the rain, perhaps not. It’s 2600nm and we could be aloft awhile, so, Master el-Quarters, victuals only, no sandbags, provisioning the giant hamper. It will be not a picnic!
Race #1886
INFOby brainaid.de
SOL Balloon PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC - SBB
Race starts: Oct 15th 11:00 Registration Open!
▶ Flash
GO TO RACE

Alicante TIMED Race 2025

Welcome to beautiful Alicante on Spain’s Costa Blanca. Dine on some local tapas and paella before racing your America’s Cup foiling winged-sail AC72 catamaran on this 208nm race out to and around Isla de Formentera returning to Alicante after rounding the mark at Cap de la Nau. There should be some very fast times! 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 #1958
INFOby brainaid.de
AC72v2 Particulars
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ4 - TRCH - SUPSOL - SYC
RACE CLOSE: Saturday,
25 October at 23:00 UTC
Race starts: Oct 12th 12:00 Registration Open!
▶ Flash
GO TO RACE

Marion Island to Fernando de Noronha 2025

The adventure continues as we launch the next to last leg of the RWW series. This leg takes us from Marion Island to Fernando de Noronha, an island in the world heritage archipelago with the same name, 220 nm off the coast of Brazil. This race doubles up as #7 of RWW and #10 of OCCH. A 4400 nm journey of endurance, strategy and pure adrenaline. The RWW series is as whole, part of the 2025 ocean championship. Get ready for this exhilarating challenge as we set sail on the third to last leg of the 2025 Ocean Championship Series! This leg will be raced aboard the Judel Vrolijk 62, pushing sailors to their limits as they navigate the open ocean. Do you have what it takes to conquer the Southern Atlantic? Join us and put your skills to the ultimate test!
PRIZE: SMPF
Race #1955
INFO by brainaid.de
JV 62 PARTICULARS    
WX Updates:
0430 / 1030 / 1630 / 2230    
Ranking: OCQ4 - RWW - OCCH - SUPSOL - SYC
Race starts: Oct 06th 11:00 Registration Closed
▶ Flash
GO TO RACE

Camel or Balloon 2025 - Khartoum to N'Djamena


As there is a lull in the calendar, your PRO has decided to risk an experiment – a balloon race over land, using a workaround to eliminate the recognition of water/land transitions. If it fails, the server may crash, and we abandon PDQ, hopefully after leg 7 of the ABS race to Suez has been decided, and before the start of the 1st OCC of Q4. Camel or Balloon – c 1000 nm from a field on the north bank of the Blue Nile at Khartoum to a field on the north bank of the Chari River at N'Djamena!
Race #1956
INFOby brainaid.de
SOL Balloon PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: none
RACE CLOSE: Sunday,
October 26 at 2300 UTC.
Race starts: Sep 28th 09:00 Registration Closed
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member vida
  2. Sailonline Yacht Club Member WRmirekd
  3. Sailonline Yacht Club Member KaSToR
  4. Sailonline Yacht Club Member CriticalHippo
  5. Sailonline Yacht Club Member rafa
  6. Sailonline Yacht Club Member Kipper1258
  7. Sailonline Yacht Club Member CollegeFund
  8. Sailonline Yacht Club Member BRENTGRAY
  9. Sailonline Yacht Club Member Sax747
  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