Gribs

[ynotjude]

Hi im new to open cpn and the gribs im uploading from the auto email... Sailgrib? Seem very un detailed and inaccurate. Im up in in lake tahoe and it was blowing 20+ today and the grib showed 6., and the grib only showed one wind arrow for the whole lake. I tried changing settings for greater detail but it didnt help. Is open cpn always this inaccurate? Thx

[ynotjude]

I should add that some peoples gribs in open cpn look more detailed and their sustem seems more setup. Should i get my gribs some place else?

[jstevens]

Not familiar with Sailgrib, but it should provide the means to specify the desity of the data points. The more dense the points the larger the download file and the longer the download period. Sounds like your download is set for a fast download rather than a detailed forecast.
That said, I've only used GRIBS for sealevel wind forecasts.

John

[nohal]

ynotjude...
The resolution of the gribs depends mostly on the model used. If you are downloading data from the most widely used GFS model created by NOAA, it likely has the default resolution of 0.5 degrees (1 degree equals to 60 nautical miles as we know from the books). It means that on Lake Tahoe, being about 20x10 miles, there is, if we are lucky, exactly 1 point calculated by the model and it is highly unlikely local geography around it is taken into account much.
The "inaccuracy" has absolutely nothing to do with OpenCPN, the program just displays what's calculated in the model and contained in the grib data you feed it.
Yes, there are other tools that calculate interpolated values between the points contained in the model and display the arrows at a constant density regardless of how far you zoom. You have to keep on mind that the feeling of higher accuracy and higher resolution they give you is pretty much completely false.
If you want accurate forecast for your area, you need to get data from a local model with much higher precision. They are usually hard to get for inland areas like lakes as the local geography is too complicated and too unimportant to be modeled properly by the agencies producing the forecasting models.

Pavel

[ynotjude]

Thx great responses. Im a capt on tahoe sailing a 65 macgregor and use windalert, windyty, and noaa for weekly outlooks and realtime for daily cruises. And yes the mountains dictate the funnels and katabatic winds...as well as microbursts. So in a place like Tahoe i usually just show up to the dock and see whats happening. My main interest is for weather routing for optimizing long circumnavigations around the lake. Are there any programs or apps for that? Or should i just stay old school? Thx for the replies.. Open cpn is just something stumbled upon im not really sure at this point what its real advantage is... Although it is free and AIS etc is nice

[nohal]

Never been to Tahoe, but live in an area with similar geography, although coastal and very, very far south. All I can say is that you can forget about the global forecasts for anything but outlook hints - if a weather system is approaching, it will most likely pass you. What it will locally look like may vary. More than that, your experience and a look at the sky to the west will always be more useful short term. For a short, say day long, forecast, I would sure add the radar data from something like South Lake Tahoe, CA Interactive Weather Radar Map - AccuWeather.com to my list. To plan a several day circumnavigation of the lake, a crystal ball is the most handy tool IMHO. Weather routing, in the sense I use it, is completely out of the question on a water body of this size and conditions

Pavel

[ynotjude]

Yea thats what i do. I read people use opencpn for racing in smaller areas and im trying to figure out how... Or why someone would use weather routing at all... For currents? Like in sf bay or what...

[nohal]

Use for racing does not necessarily mean weather routing, you can also plan your waypoints and navigate to them or look at your opponents on AIS or...
Especially on sailing boats, pretty much everybody does weather routing all the time, what differs are the data and tools used. And they differ a lot - from v1 eyeball and a little common sense to computer aided "scientific" route planning using digital weather models.

The kind of grib data that made you start this thread, for example, is good to do weather routing from SF to Hawaii, but in SF bay, Tahoe or other small water body, it is of course pretty much useless for similar purposes and local knowledge and current meteo situation are far superior to plan your route.

If you can get Grib data with resolution better than say 0.1 deg, they start to be useful even in more constrained areas (Still I have to see one really usable on a lake in the middle of the mountains, but maybe it does exist, or will)

To give you an example:
This is the output of a local Aladin model for the north of Adriatic:

Note that this whole model covers an area of about 1x1 deg.
The global GFS model calculates only 16 points for this whole area at it's very best resolution of 0.25 degrees and gives you something like this:

Of course you have to keep on mind that even the local model is just as good as the historical data it is based on and the math implementing it - it is definitely not going to steer the boat for you and the conditions you meet on the water may and will differ.

As to your "why?" Because they can and because they want. It does not mean you should or have to or that you are weird if you don't.

Pavel

[rgleason]

If you have nmea data available, try Tactic_pi plugin for racing.

[ynotjude]

Great thx so much. That should be enough to get me going... I appreciated the detailed and informative responses.

[ynotjude]

So Im following up with this and a new problem. Ive found some gribs that work well at 12kms (whatever that is) and have many more wind arrows around lake tahoe. But , now Im trying to weather route (for practice) and my polar keeps failing. Im not sure why i uploaded a generic polar for a columbia 50 even though i have a 26 and figure id modify it later, I just want to have it working so i can continue to setup my cpn....why is my polar not working?

thanks!!

[admiralslater]

12 kms (kilometers = 8 miles give or take

[rgleason]

Ynotjude. You should be posting to the weather_routing thread probably, but there are a couple of things you could do. Make sure the start time is set for the beginning of the grib file or that you have a grib file that is calid for the start time. Also the problem might be having a valid polar file. Try several different ones. Add a couple of polars to the list, select one and see if you can view the pilar file and then run it.

[rgleason]

Ps also read past posts in that thread.

[barnakiel]

I think plain banana gives you .25 data.

Why do you want more accurate data? You can't see with your eyes, ears and the cera what the wx is doing?

b.