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I'm having some trouble reading a WRF weather forecast prediction file where the coordinates are in a custom Lambert Conformal Conical projection.

My final goal is to get the weather forecast a a certain location given in a WGS84 projection (lat/lon), which I know for sure is within the NetCDF file.

I'm using a Python script and the NetCDF4 and PyProj libraries.

In the NetCDF file, downloaded from the local weather forecast service, there is the following information about the used projection:

<class 'netCDF4._netCDF4.Variable'>
int32 Lambert_Conformal()
    grid_mapping_name: lambert_conformal_conic
    longitude_of_central_meridian: -14.100000381469727
    standard_parallel: [43. 43.]
    latitude_of_projection_origin: 40.71900177001953
    false_easting: -241.44951
    false_northing: -4.10447
unlimited dimensions: 
current shape = ()
filling on, default _FillValue of -2147483647 used

So I try to recreate this using the pyproj library:

lat_0 = variables['Lambert_Conformal'].latitude_of_projection_origin
lon_0 = variables['Lambert_Conformal'].longitude_of_central_meridian
lat_1 = variables['Lambert_Conformal'].standard_parallel[0]
lat_2 = variables['Lambert_Conformal'].standard_parallel[1]
x_0 = variables['Lambert_Conformal'].false_easting
y_0 = variables['Lambert_Conformal'].false_northing

I create a LCC origin projection with PyProj, as well as the destination WSG84 projection:

lcc = Proj(proj='lcc' , lat_1=lat_1, lat_2=lat_2,
           lat_0=lat_0, lon_0=lon_0, x_0=x_0, y_0=y_0,
           a=6370000, b=6370000)
# Resulting projection string: '+proj=lcc +lat_0=40.7190017700195 +lon_0=-14.1000003814697 +lat_1=43 +lat_2=43 +x_0=-241.44951 +y_0=-4.10447 +R=6370000 +units=m +no_defs'

wgs84 = Proj(proj='latlong', datum='WGS84')

Now that I have everything set-up, I perform the actual conversion between projections:

transformer = Transformer.from_proj(proj_from=wgs84, proj_to=lcc)

# Geographical point which I know is contained within the weather forecast.
lat_geo = 42.666981
lon_geo = -8.006540

coords=transformer.transform(lon_geo,lat_geo)

As result, I get:

(497466.92765635485, 234691.1045065646)

which makes no sense to me, as I was expected something contained within the range of the variables in NetCDF file. For reference, the latitude variable in the NetCDF file is between 40.719 and 40.44141.

So, what am I missing here?

  • Are the lat_0, lat_1, lat_2... parameters correctly loaded? Did I misplaced some of them?
  • Is there some missing projection information in NetCDF file?
  • I see that there is actually a single standard parallel (the same, but repeated twice). Does that make sense?

TLDR: How do I go from this custom LCC projection to standard WGS84 coordinates?

More information:

  • The weather forecast agency says that the global lon/lat coordinates are supossed to be:
GeospatialCoverage:

    Longitude: -11.261331695726343 to -5.101928395629246 degrees_east
    Latitude: 40.421302033295966 to 45.230011083267385 degrees_north
  • This is a map using their available web tools:

Weather forecast domain

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    did you try with coords=transformer.transform(lat_geo,lon_geo)? – Ian Turton Dec 18 '20 at 17:47
  • Yes, I did try inverting lat_geo and lon_geo. If I do that I get (8295902.637445953, -3265084.29720419) , which is still far from correct. Also, I double check by feeding the projection string to QGIS and I got that the correct order seems to be longitude first, latitude later. So, everything points to the projection string and the projection parameters... – QuestionsAboutWeather Dec 18 '20 at 17:59
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    I might be completely wrong, but the false_easting and false_northing look odd to me. Commonly they are in units of meters in the projected system, but numerically, -241.44951 and -4.10447 look more like degrees. – Dave X Dec 18 '20 at 18:09
  • Thanks, @DaveX. It does look akward indeed. Thank you for your suggestion. But I found the problem. I'll update the question – QuestionsAboutWeather Dec 20 '20 at 19:38
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Well... I found the solution to my problem. The coordinates were always in geodetic coordinates. I didn't realise because I was misreading the variables['lat'] and variables['lon'] arrays. Also, I was mislead by the specification of a Lambert Conformal Conic projection.

Lessons learned:

  • Always check the data contained within your file.
  • Lots of experience gained with the pyproj library.

Thank you everyone for your time.

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