grb file details

[knoekvansoest]

Hi all,

I routinely download huge grib files from the dutch meteo office and convert these to much smaller files containing just the variables that are of interest to me.

This works fine for wind, air temp and air pressure; the info is recognized by opencpn grib plugin and displayed as expected.

However, I am having trouble to figure out what the requirements are in order for opencpn to recognize information on e.g. wind gusts and rainfall. The information is in the huge files, but I cannot get opencpn grib plugin to recognize these variables.

Can someone point out where I can find detailed information on the grb file requirements in order to have opencpn grib plugin recognize the info?

any help appreciated!

regards knoek van soest

[Auspicious]

https://lmgtfy.com/?q=opencpn+grib

First response is the OpenCPN documentation for overlaying grib information.

[knoekvansoest]

Ok so maybe my question was not clear ... I have a grb file provided by the dutch meteo office. I have confirmed personally that this file contains info on wind gusts, precipitation, cloud cover. But these variables are not shown in the grib plugin. Not sure why. At this point, I suspect that this is because the field codes (parameter numbers etc) in the grb files do not match what opencpn grib plugin expects. I am aware of the standard documentation, but in that I have not found detailed information on the parameters that opencpn grib plugin expects in the grb files. Once I know this it is straightforward (using a small python prog) to transform the data in the source grb file to a target grb file that opencpn grib plugin will understand. Thus I am looking for low level details on the structure of the grb files that opencpn grib plugin expects.

[Rasbats]

knoekvansoest...

Two files here may help:

https://github.com/OpenCPN/OpenCPN/t...ns/grib_pi/src

The files are:
GribV1Record.cpp
GribV2Record.cpp

This will tell you the parameters used by OpenCPN.

Mike

[knoekvansoest]

Originally Posted by Rasbats

knoekvansoest...

Two files here may help:

https://github.com/OpenCPN/OpenCPN/t...ns/grib_pi/src

The files are:
GribV1Record.cpp
GribV2Record.cpp

This will tell you the parameters used by OpenCPN.

Mike

Excellent! I am sure that will enable me to sort this out. Thanks Mike!

[did-g]

Originally Posted by knoekvansoest

Excellent! I am sure that will enable me to sort this out. Thanks Mike!

Is a sample available? At least with center number?

For gust it's likely an issue with the level 0, 2m or 10m.

For cloud and rain there's big if else or a switch on center number.
If dutch center is not in, it's unlikely defaults are going to work.

Wind, pressure an temp are more standard an work out of the box most of the time.

[knoekvansoest]

Originally Posted by did-g

Is a sample available? At least with center number?

For gust it's likely an issue with the level 0, 2m or 10m.

For cloud and rain there's big if else or a switch on center number.
If dutch center is not in, it's unlikely defaults are going to work.

Wind, pressure an temp are more standard an work out of the box most of the time.

Dutch meteo office (KNMI) is indeed one of the options in a big if else in the c code. Didn't have time yet to look into this in detail, but my idea at this point is to tweak the grb file that I can download from the meteo office (which I tweak anyways to reduce size) to make it digestible to the existing c code for the grib plugin.

The obvious alternative would be to update the c code of the grib plugin in a future release, to make it suitable to deal with current KNMI files ...

knoek van soest

[knoekvansoest]

Update:

this was more involved than I (as a complete newbie on the details of gribfiles, accessed through pygrib) anticipated, but in the end I have figured out (per parameter) which grib keys had to be changed in the KNMI grib messages in order to make opencpn grib plugin accept them. I achieved this by comparing key/value pairs with a grib file from another source that was loaded ok in opencpn. I now have access to wind, wind gusts, pressure, rainfall, cloud cover and air temp, which is what I intended!

Happy sailing and thanks for your help,
Knoek