I have some grib files from ECMWF ERA-Interim Reanalysis in which all the data are projected onto the N128 Gaussian grid (256 points of latitude x 512 points of longitude, https://www.ecmwf.int/en/forecasts/documentation-and-support/gaussian_n128).

I need to compare these data with some that have been geo-located with a GPS and are therefore in WGS84 latitude and longitude.

How should I do?

Is there a reprojection to do?

If so, which one?

I am googling like crazy but I can't find any clear explanation.

  • Oh, how exciting (not). It's a quasi-projected system something like Sinusoidal where the length of the parallel is reduced as you move away from the equator. AKA equator is 512 cells long, near the pole, it's 18 cells. A cell is approximately 0.7°x0.7°. Maybe it's integerized Sinusoidal but I don't know. – mkennedy Jan 15 at 4:41
  • ...yes, this was clear also from the description of the gaussian grid, but how does it relate to a projected one like WGS84? Should I take the coordinates "as-is" and slap them onto my WGS84 grid? Should I transform them somehow? What's the point of the sarcasm considering that the question was not really clearly answered? – Mutewinter Jan 16 at 16:46
  • There is a convertion to do, from a Reduced Gaussian to a regular grid. In my searches, cdo seems to do that: cdo setgridtype,regular ifile ofile. Also wgrib2: wgrib2 -reduced_gaussian_grid OUT -1 INTERPOLATION – Gabriel De Luca Jan 18 at 7:03
  • Also, did you try to import into QGIS? GDAL GRIB Driver supports georeferencing. – Gabriel De Luca Jan 18 at 7:12
  • @GabrielDeLuca you are right but it does not seem to be a reduced Gaussian grid, or at least, any attempt to use cdo/wgrib to convert to a regular grid failed stating that there were no information regarding the reduced grid. I assumed it was a regular Gaussian grid and did a point to point comparison with WGS84 Lat-Lon – Mutewinter Jan 24 at 11:02

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