Processing data for addVelocity

[jmhatch]

I'm not sure this is a leaflet.extras2 issue, but I thought I'd start here. I'm trying to add wind data to a leaflet map. I download the data using the following link,

https://nomads.ncep.noaa.gov/cgi-bin/filter_gfs_1p00.pl?file=gfs.t06z.pgrb2.1p00.f000&lev_10_m_above_ground=on&var_UGRD=on&var_VGRD=on&subregion=&leftlon=-72.74273&rightlon=-67.92787&toplat=43.64460&bottomlat=40.04384&dir=%2Fgfs.20201229%2F06

Then I convert the grib file to json using the weacast-grib2json program,

grib2json -d -n -o gfs.t06z.pgrb2.1p00.f000 gfs.t06z.pgrb2.1p00.f000.json

Basically following the guidance found here and here.

However, when I display the wind data on my leaflet map,

leaflet() %>%
    setView(lng = -70.3361, lat = 41.8451, zoom = 10) %>%
    addProviderTiles(providers$Esri.WorldImagery) %>%
    addVelocity(content = 'gfs.t06z.pgrb2.1p00.f000.json')

it looks different than what is displayed on windy or earth (the angle of the particles looks off). Not sure what is going on...

Windy

vs. mine

Maybe it's a projection issue?

[trafficonese]

Hey @jmhatch, that is very interesting indeed. Unfortunately I have no idea, where this "bug" is coming from. I will have a look at my code but I'm pretty sure I just pass the json to the underlying plugin-library. So no transformation is happening on my side.

Are you sure that you were looking at the same timeranges? I guess a time difference of 1 hour can already make a huge difference.

[jmhatch]

@trafficonese I'm pretty sure it's the same time range. Here is another example, with a ref time of 2021-02-09T06:00:00.000Z from the json file I downloaded, which seems to correspond with what Windy is reporting for GFS.

Windy

vs. mine

The discrepancy is pretty apparent. I must be doing something wrong?

[trafficonese]

Could you maybe try to use the json file with the underlying JS-library?
Unfortunately, I am not able to install grib2json on my computer, so its a bit hard for me to debug..

[jmhatch]

Good idea @trafficonese!

I was able to replicate the 'mine' image above using the leaflet-velocity JS library. So it isn't a leaflet.extras2 issue. It must be a data issue. I'll open up an issue over at leaflet-velocity to see if someone can help me out.

Thanks!