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Board » Technical Discussion » Weather data compression and resolution

Posted by
hmm


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2011-11-03 13:35:22
2015-02-02 11:36:05 - Weather data compression and resolution
One of the unseen changes this year is that the weather data from server to client is now compressed. (Actually all data transfer which was not yet compressed is now)

In practice this means, that the size of the weather data transferred over network has decreased by about 60%.

Because of this, for the SWR Leg 4 from Sanya to Auckland we are using higher 1 degree weather resolution (the wind data points are in 1x1 degree grid).

Previously we have usually used 2 degree resolution for ocean races, so this means we have 4 times more weather data. I hope this change adds to the accuracy of the simulation by picking smaller changes in the weather more easily.

In practice, the transferred weather size has increased to 1.6 times that it would have been previously. (compressed data is 40% of uncompressed size but data is now 4 times bigger, 0.4*4 = 1.6).

The time for loading the weather data is then bit longer than before, but I hope that the finer resolution outweights that. After all the weather data is loaded only once in starting of the client and then once every 6 hours.

As Leg4 is now open for practice, please give your feedback about the change, both negative and positive.
Posted by
Zembu


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2008-12-15 20:58:27
2015-02-02 14:29:42 - Re: Weather data compression and resolution
Hi hmm, I am wondering if the change is the reason that now for Sanya to Aukland, when loading the grib into Bluewater, the following report appears;
"The following errors occurred while loading C:/SOL/Races/
weather_100_global_gfs_20150201_1627.grb Attempt to add Grib Sequencies anyway? "Only read 18904 out of 19068 bytes for EntireMsg (offs=1029672)""???
Posted by
hmm


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2011-11-03 13:35:22
2015-02-02 17:52:08 - Re: Weather data compression and resolution
I would more guess that the problem is the weather data in the grib crossing international date line, but I am no expert in BWR as I don't use a router..
Posted by
ita10267


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2008-12-29 10:33:09
2015-02-05 09:42:17 - Re: Weather data compression and resolution
yes,
the problem is that gribs produced on brainaid site are defined to be continous when crossing greenwich, so they present discountinuity in Longitude on the IDL (transition from -180 to 180). Bluewather and some other grib viewing softwares don't know how to handle this. If you fetch gribs directly from NOAA you can "tweak" this by moving the discontinuity to Greenwich (transition from 0 to 360) in order to cross the IDL with no problems.

hope that this clarifies the matter

--- Last Edited by ita10267 at 2015-02-05 09:42:36 ---
Posted by
Zembu


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2015-02-08 23:03:19 - Re: Weather data compression and resolution
ITA, Yes, I am aware of the fact that Bluewater needs to have part of the grib "disabled", when the grib straddles the dateline. This is not a problem and I do not think that it is the reason why Bluewater signals that the file is smaller than expected. There is no need to change the discontinuity to Greenwich. One merely uses the grib manager in Bluewater to not use the unwanted part for routing. When I have used Bluewater in the past when the grib straddles the dateline, I don't recall noticing the following from Bluewater;
"The following errors occurred while loading C:/SOL/Races/
weather_100_global_gfs_20150201_1627.grb Attempt to add Grib Sequencies anyway? "Only read 18904 out of 19068 bytes for EntireMsg (offs=1029672)""???

Further to the above, as of approximately 10th/11th Feb 2015, the above message is no longer displayed by Bluewater. From discussion noted in the chat, I surmise that Brainaid has fixed the problem.
--- Last Edited by Zembu at 2015-02-09 17:02:42 ---

--- Last Edited by Zembu at 2015-02-11 17:06:30 ---
Posted by
Kipper1258


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2016-03-17 12:14:53 - Re: Weather data compression and resolution
ita10267 said @ 2015-02-05 09:42:17:
If you fetch gribs directly from NOAA ...

ita, I'm wondering if you have the details on how to do that. I've had a look at the NOAA site but would not know where to start. Is there a simple way to pull the gribs from NOAA or a script available to do that?

Thanks

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