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Old 10-11-2017, 11:51   #1
Moo
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Inverted GRIB display

Discovered this one today.

I opened a GRIB file (an ECMWF file downloaded with TZ Navigator v3) which I generally use to access and view/analyse the very good Euro weather model.

When I open this in OCPN the GRIB display is inverted north to south. I have tried numerous file and see the same result in all from this source.

Can others confirm this please? Any files from other sources affected the same way?

Please see attached screenshots and sample file (remove .avi extension).
Attached Thumbnails
Click image for larger version

Name:	ECM GRIB TZ.jpg
Views:	217
Size:	158.0 KB
ID:	159022   Click image for larger version

Name:	GFS GRIB Saildocs.jpg
Views:	228
Size:	153.0 KB
ID:	159023  

Attached Files
There was an error processing this video...
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Old 10-11-2017, 12:19   #2
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Re: Inverted GRIB display

I'm not sure what we're looking at here. Both images are OpenCPN and, according to the file reference bottom left, are two different GRIB files. It does not appear to be an inversion, more like a day apart as the wind barbs are generally in the same direction?
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Old 10-11-2017, 12:21   #3
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Re: Inverted GRIB display

Quote:
Originally Posted by hoolie View Post
I'm not sure what we're looking at here. Both images are OpenCPN and, according to the file reference bottom left, are two different GRIB files. It does not appear to be an inversion, more like a day apart as the wind barbs are generally in the same direction?
It is an inversion.. Same day, same time, same generation time.
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Old 10-11-2017, 12:34   #4
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Re: Inverted GRIB display

It's not an inversion as the wind barbs are generally similar and the isobars are shaped differently, even if they were inverted. It doesn't look like an OpenCPN problem from the images you've shown. How do they look on a different scale?
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Old 10-11-2017, 12:37   #5
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Re: Inverted GRIB display

trying to post the file again . remove .pdf
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Old 10-11-2017, 12:42   #6
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Re: Inverted GRIB display

Fek it the forum keeps saying that the file is an invalid video when I post it with .pdf extension.

Anyway hoolie here is the same file displayed in TZ Navigator. Can you see that it is inverted yet?
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Name:	TZ same ECM.jpg
Views:	151
Size:	192.5 KB
ID:	159029  
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Old 10-11-2017, 13:01   #7
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Re: Inverted GRIB display

I think we're talking at cross-purposes. By "inverted" I assumed you meant "upside-down". They're not.

The wind barbs are very similar and generally point in the same direction but the isobar low is in a different place. This is what I would expect if you have a time shift between the two GRIB files. Agreed the shape is similar if it was inverted but by no means an accurate mirror image.

You've shown a TZ image for one of the GRIB files. What does it look like with the other?
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Old 10-11-2017, 13:08   #8
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Re: Inverted GRIB display

Quote:
Originally Posted by hoolie View Post
I think we're talking at cross-purposes. By "inverted" I assumed you meant "upside-down". They're not.

The wind barbs are very similar and generally point in the same direction but the isobar low is in a different place. This is what I would expect if you have a time shift between the two GRIB files. Agreed the shape is similar if it was inverted but by no means an accurate mirror image.

You've shown a TZ image for one of the GRIB files. What does it look like with the other?
Yes agreed some misunderstanding.. The isobars are inverted.

If there was a time shift the wind arrows would change direction to match the isobars

Here is the GFS in TZ Navigator it displays correctly.

I know TZ Navigator v3 handles both file formats correctly as I usually compare GFS from Saildocs with ECM from Timezero when on passage.
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Name:	TZ GFS GRIB.jpg
Views:	121
Size:	213.7 KB
ID:	159030  
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Old 10-11-2017, 13:18   #9
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Re: Inverted GRIB display

One more try with the GRIB file. replace .pdf with .grb.bz2
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Old 10-11-2017, 13:42   #10
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Re: Inverted GRIB display

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Originally Posted by Moo View Post
One more try with the GRIB file. replace .pdf with .grb.bz2
How big is it?

Likely I know what's going on but it'd be a lot easier with a grib file for testing.
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Old 10-11-2017, 13:49   #11
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Re: Inverted GRIB display

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Originally Posted by did-g View Post
How big is it?

Likely I know what's going on but it'd be a lot easier with a grib file for testing.
500k.. I have tried with a smaller, and bigger, files but I keep getting the same result from the forum. Can you PM me an email address and I can send the file to you.
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Old 10-11-2017, 13:52   #12
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Re: Inverted GRIB display

I agree there's a difference with the OCPN ECM file image but it's certainly not simply an inversion. For example the winds off the NE coast of NZ NI are quite different on that GRIB compared with the other three images.

I still think it's a time shift problem. What is the time span of the ECM GRIB file? Maybe OCPN is picking up incorrectly timed data? Can you try other dates/times from that GRIB file with TZ to see whether you can match the OCPN image?
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Old 10-11-2017, 13:57   #13
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Re: Inverted GRIB display

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Originally Posted by hoolie View Post
I agree there's a difference with the OCPN ECM file image but it's certainly not simply an inversion. For example the winds off the NE coast of NZ NI are quite different on that GRIB compared with the other three images.

I still think it's a time shift problem. What is the time span of the ECM GRIB file? Maybe OCPN is picking up incorrectly timed data? Can you try other dates/times from that GRIB file with TZ to see whether you can match the OCPN image?
Have a think about which direction the air circulation is around a LP in the southern hemisphere, then look again.
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Old 10-11-2017, 14:05   #14
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Re: Inverted GRIB display

I know the physics! Three of your images show 40kn north north west wind off the NE coast of NZ NI. The ECM OCPN you're complaining about shows 10kn south easterly
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Old 10-11-2017, 14:16   #15
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Re: Inverted GRIB display

Quote:
Originally Posted by hoolie View Post
I agree there's a difference with the OCPN ECM file image but it's certainly not simply an inversion. For example the winds off the NE coast of NZ NI are quite different on that GRIB compared with the other three images.

I still think it's a time shift problem. What is the time span of the ECM GRIB file? Maybe OCPN is picking up incorrectly timed data? Can you try other dates/times from that GRIB file with TZ to see whether you can match the OCPN image?
There's various way you can read grib grid data, left to right, right to left, top to bottom, bottom to to top and so on. If ECM files are using an order we've never seen before OCPN could be 'confused'.
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