I'm relatively new to NetCDF and working with a CMAQ model output in NetCDF format. There is no explicit lat/long variable, rather each variable is stored with a ROW/COL. I assume the Global attributes contain what I need to convert to long/lat or Northings and Eastings but I have not found a way to do it with the netCDF4 package or Xarray.

The files are 20GB so I'll just include the start of the metadata

print (nc_f)
<class 'netCDF4._netCDF4.Dataset'>
root group (NETCDF3_CLASSIC data model, file format NETCDF3):
    IOAPI_VERSION: $Id: @(#) ioapi library version 3.1 $                                           
    EXEC_ID: CCTM_v521.exe                                                                   
    FTYPE: 1
    CDATE: 2018111
    CTIME: 95359
    WDATE: 2018111
    WTIME: 95359
    SDATE: 2013001
    STIME: 10000
    TSTEP: 10000
    NTHIK: 1
    NCOLS: 201
    NROWS: 306
    NLAYS: 1
    NVARS: 112
    GDTYP: 2
    P_ALP: 30.0
    P_BET: 60.0
    P_GAM: -121.0
    XCENT: -121.0
    YCENT: 49.0
    XORIG: 12000.0
    YORIG: 24000.0
    XCELL: 4000.0
    YCELL: 4000.0
    VGTYP: 7
    VGTOP: 10000.0
    VGLVLS: [1.     0.9975]
    GDNAM: 04km            
    UPNAM: M3CPLE          
    VAR-LIST: NO2             NO              O3              NO3             N2O5            HNO3            HONO            H2O2            NTROH           NTRALK          ROOH            ALD2            ALDX            ISOPX           IEPOX           FORM            FACD            CO              AACD            MEPX            MEOH            PAN             PACD            PANX            CRPX            OPAN            NTRM            NTRI            SO2             SULF            CL2             HOCL            FMCL            HCL             CLNO2           MAPAN           NTRCN           NTRCNOH         NTRPX           FORM_PRIMARY    ALD2_PRIMARY    HG              HGIIGAS         ASO4J           ASO4I           ANH4J           ANH4I           ANO3J           ANO3I           AALK1J          AALK2J          AXYL1J          AXYL2J          AXYL3J          ATOL1J          ATOL2J          ATOL3J          ABNZ1J          ABNZ2J          ABNZ3J          APAH1J          APAH2J          APAH3J          ATRP1J          ATRP2J          AISO1J          AISO2J          ASQTJ           AORGCJ          APOCJ           APOCI           APNCOMJ         APNCOMI         AECJ            AECI            AOTHRJ          AFEJ            AALJ            ASIJ            ATIJ            ACAJ            AMGJ            AKJ             AMNJ            ACORS           ASOIL           ANAJ            ACLJ            ASEACAT         ACLK            ASO4K           ANH4K           ANO3K           AISO3J          AOLGAJ          AOLGBJ          NH3             SV_ALK1         SV_ALK2         SV_XYL1         SV_XYL2         SV_TOL1         SV_TOL2         SV_BNZ1         SV_BNZ2         SV_PAH1         SV_PAH2         SV_TRP1         SV_TRP2         SV_ISO1         SV_ISO2         SV_SQT          
    FILEDESC: hourly 1-layer cross-point RADM dry deposition data                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
    dimensions(sizes): TSTEP(744), DATE-TIME(2), LAY(1), VAR(112), ROW(306), COL(201)
    variables(dimensions): int32 TFLAG(TSTEP,VAR,DATE-TIME), float32 NO2(TSTEP,LAY,ROW,COL)

It's a 4km square grid that appears to start at 121W and 49N, It's not parallel to latitudes and I assume that "VGLVLS: [1. 0.9975]" would contain the angle but I have no idea what python functions to use to convert.

Looking at other questions it seemed there was a built in function as below but there is of course no longitude_in key in my dataset.

    # Read variables from ncdf
    lon = nc.variables['longitude_in'][:]
    lat = nc.variables['latitude_in'][:]

That's from this solution but it doesn't work on this dataset: Longitude/Latitude from NetCDF with (row,column)

Ideally I'd like to get LAT and LONG of the center of each square (although knowing how to get the four corners would be good too) each in a similar 2D array so I can output a .csv file with ROW, COL, LAT, LONG, NO2, SO2, O3, NO3.....after reducing the parameter count and summing the variable data over the month (I have the rest of it figured out I think).

  • I suspect the attributes P_ALP, P_BET, P_GAM describe a coordinate projection, but unless you can see something that says if its conical or cylindrical or transverse mercator you might be stuck. The XCENT, YCENT, XORIG, YORIG look like they describe how to convert row and column to coordinates in that coordinate system but getting them into lat-long needs the full projection info beyond the alpha, beta, gamma parameters because we don't know what they mean (and I'm guessing a bit anyway). Where's the source documentation? Can we see it?
    – Spacedman
    Commented Mar 15, 2021 at 13:53
  • Unfortunately there is no documentation beyond the files themselves, these were created by a previous modeler in 2014 or so. I believe CMAQ uses a 6370000 spheroid radius and I'd like to get the output in a WGS84 datum but you're right, I don't see anything in the file that would tell me the original datum.
    – RichardS
    Commented Mar 15, 2021 at 17:28
  • I've been attempting to find more on this across the web; CMAS sites, IOAPI etc. It seems that P_ALP is the first true latitude, P_BET is the second, and P_GAM is the origin longitude. GDTYP is the projection and 2 specifically is LCC. The VG Stuff is for vertical layers, but this is surface only data so not sure why it's included. It also looks like these were extracted in 2018 (WDATE) which is later than I expected.
    – RichardS
    Commented Mar 17, 2021 at 1:12

1 Answer 1


I'm going to do this in R because that's easiest for me. But if it works then this should be translatable to Python, and its possible that all you need is the PROJ string that I'm going to construct...

From those data I'd construct a proj string that looks like this:

projstring = "+ellps=WGS84  +proj=lcc +units=m +x_0=12000 +y_0=24000   +lat_1=30 +lat_2=60 +lon_0=-121 +lat_0=49.0"
###            earth shape     GDTYP2          x and y false origin    standard parallels  projection centre
### https://proj-tmp.readthedocs.io/en/docs/operations/projections/lcc.html

individual parts are annotated in the second line and the descriptions of the parameters are in the link. Now I'm not sure if the false origins are correct and your grid coordinates start at (0,0), or if your grid starts at 12000,24000 and the false origins should be 0 and 0. Things should end up in more or less the same place but that's one thing that might not be right.

Also, the LCC projection requires two latitudes and a lat-long centre, but you've got two longitudes (P_GAM=-121.0 and XCENT=-121.0). They're the same value, but it might indicate that something else is going on. Not sure.

Anyway, now we have that string we can construct a grid of (NCOLSxNROWS) points with separation XCELL, YCELL (4000 x 4000) starting at (0,0):

grid = expand.grid(x = seq(0, by=XCELL, len=NCOLS),
                   y = seq(0, by=YCELL, len=NROWS))

> head(grid)
      x y
1     0 0
2  4000 0
3  8000 0

Then make that an R spatial object with the coordinate system given by that proj string:

gridsf = st_as_sf(grid, coords=1:2, crs=projstring)

and transform to lat-long (epsg coordinate code 4326):

gridll = st_transform(gridsf, "epsg:4326")

That produces a set of points that form this grid over Canada:

enter image description here

If you know where this grid should be then this might be correct. Or it might be that the offsets are off and you should try it the other way round.

Also, this might be the grid centre points or the grid cell bottom-left coordinates.

Also also, my assumption of the WGS84 ellipse might be wrong. If you can find out that your data comes from a model that assumes a spherical earth of radius 6300000m then you can make the proj string say that with some other parameters.

Ideally you should be able to go back to the community that generated this data and get precise information about the coordinate system used and perhaps a ready-computed grid of lat-long point coordinates. But hey, back in the real world...

Anyway, that proj string should be usable in a Python package that does PROJ transformations, like pyproj or GDAL, or PyQGIS, to transform from your grid to lat-long.

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