Sailonline.org - Online Sailing

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!

Tweets by @sailonline

Board » General Discussion » SOL maps Datum & Interpolations.

Posted by JB 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 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 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+0.5cos(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 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 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 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 Wind_Interpolations.pdf (113.1 KB)
Posted by kroppyer 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:

Melbourne to Osaka Prelude 2024

Welcome once again to what these days is Sailonline’s almost annual virtual Melbourne to Osaka Yacht Race. In real life, this double-handed 5500 nm race between these two sister cities, one deep in the southern hemisphere, the other high in the northern hemisphere, is run every four to five years, and is planned to be held again in 2025, so this race is a Prelude in partnership with the Melbourne Osaka Cup 2025 organising committee collaborating with the Ocean Racing Club of Victoria (ORCV), and the Sandringham (SYC) and Osaka Hokko (OHYC) yacht clubs. On this occasion, we’ll be racing the well-known First 40, a popular size of boat for a long-distance double-handed race. With the doldrums unavoidably lying across the track, you can expect to be at virtual sea for at least a month!
Race #1669
INFO by brainaid.de
First 40 Particulars
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC
Race starts: Apr 20th 00:00 Registration Open!

▶ Flash

GO TO RACE

St Nazaire Chasse TIMED Race 2024

Welcome to a "chasse" (hunt) from St. Nazaire to La Baule on the west coast of France! This race was designed by SOLer FR_flouflou in 2010. The KER 40 was introduced to SOL in 2016 by SOLer psail, and the Sol-KER was a welcome addition to SOL’s fleet of 40- footers, as it was to the IRL Ker fleet of eponymous - like KERonimo, KukuKERchu and Ice BreaKER. This is a TIMEDrace, so you may RE-REGISTER HERE to try again, after finishing a run. You will have 13 days and 11 hours to show your skill and decision making after the race opens.
Race #1797
INFOby brainaid.de
Ker 40 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ2 - TRCH - SUPSOL - SYC
RACE CLOSE: Saturday,
27 April at 23:00 UTC
Race starts: Apr 14th 12:00 Registration Open!

▶ Flash

GO TO RACE

The Beagle in the Straits of Magellan 2024

Both before and after visiting the Falkland Islands, the Beagle extensively explored the south eastern coast of South America, hither and thither, from north of the Rio Plate to the tip of Tierra del Fuego, but it was not till the end of June 1834 that the ship made it into the Pacific Ocean, transiting via the Straits of Magellan. Online in 2024, the choice is yours - passage the strait or round the cape; 400nm or more in your Class B Tall Ship.
Race #1753
INFOby brainaid.de
Tall Ship Class B PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SVF - SYC
Race starts: Apr 09th 13:00 Registration Closed

▶ Flash

GO TO RACE

Cape Town to Auckland 2024

Welcome to the second leg of this Round The World series 2024. It's also the April edition of this year's ocean race championship. The course is the same as the one sailed in 2023, but this year we sail the iconic Swan 65, as suggested in the concluding RTW race last year.
Prepare yourselves for an epic 30-day journey, navigating through the unpredictable waters of the South Seas. It's essential to take care of provisioning to ensure a smooth and enjoyable race experience. With the longer duration, we anticipate plenty of opportunities for camaraderie, competition, and unforgettable memories.
PRIZE: SMPF
Race# 1789
INFO from brainaid.de
Swan 65 PARTICULARS
WX updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ2 - RTW - OCCH - SUPSOL - SYC
Race starts: Apr 01st 11:00 Registration Closed

▶ Flash

GO TO RACE

Go to race archive

SYC Ranking

View full list

Series

Mobile Client

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

The mobile client