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The native horizontal spatial reference for ERA Interim data is a T255 reduced Gaussian grid, equivalent to a horizontal resolution of about 80km or 0.75deg. The native file format for ERA-Interim is GRIB, with all parameters stored in GRIB1 format.

When you download ERA-Interim data you can specify the output data to be in GRIB format or in NetCDF format. This determines your options for the horizontal resolution:

If you specify GRIB as output format you will get a GRIB1 or GRIB2 file (depending on the parameter). By default the output will be on the native T255 grid, but you can specify a different Gaussian grid or a lat/long grid:
If you specify a Gaussian grid other than the default T255 the data will be interpolated to your chosen resolution. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
If you specify a lat/lon grid the data will be interpolated to your chosen resolution.. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). The lat/long equivalent of T255 is 0.703125 deg (360/(2*(255+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.75x0.75 deg.
If you specify NetCDF as output format: Our NetCDF implementation only supports regular grids, so when you extract data it is automatically interpolated from the native Gaussian grid to lat/long. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). Regarding resolution, the lat/long equivalent of T255 is 0.703125 deg (360/(2*(255+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.75x0.75 deg.

For ERA-Interim data in all representations the assumed underlying earth model (the geodetic datum) is a sphere with with radius 6367.47km. This applies to ERA-Interim data in all supplied file formats (GRIB1, GRIB2, and NetCDF) and in both grid types (Gaussian and lat/lon). Longitudes range from 0 to 360, which is equivalent to -180 to +180 in Geographic coordinate systems.

To visualise the data many software applications by default plot regular lat/lon data as a continuous surface. However, you might prefer to think of the data as point data with a regular spacing. In global data the 'top left' point will be at Longitude=0 ; Latitude=90, with further grid points spaced according to your selected resolution r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90. For example, if you download ERA-Interim data with a 0.75 degree Lat/Lon grid and plot it onto a satellite image with 0.25 degree resolution:

For GIS users:

Some software applications do not recognise the spatial reference information embedded in the data file and may require you to manually assign a spatial reference. In this case use a 6367.47km sphere for all ERA-Interim data if possible.

In practice, considering that global meteorological models operate in spatial resolutions of at least multiple kilometres and with significant spatial uncertainty, for most users assigning one of the commonly used geodetic datums (WGS1984, ETRS1989, etc.) to the downloaded data is a 'good enough' solution.

For users of GIS software, The GRIB1 sphere does not have an EPSG code.

Background information:

The different file formats handle spatial reference information differently:

GRIB1 only allows two possibilities - a spherical earth with radius 6367.47 km or an oblate spheroid earth with the major axis 6378.160 km, minor axis 6356.775 km and f = 1/297.0, as specified in the WMO GRIB Edition 1 specifications.
GRIB2 allows additional spheroids, including custom ones. For more information see the WMO GRIB2 specifications, section 2.2.1 .
ERA-Interim data provided in NetCDF format is converted on demand from the native GRIB format to NetCDF, so the spatial reference is inherited from the GRIB file.

At ECMWF the ERA-Interim data is produced and stored as a set of grid points on a reduced Gaussian grid on a sphere with radius 6367.47 km, as specified in the WMO GRIB Edition 1 specifications.

Note that other ERA products have different resolution:

ERA-40 has a resolution of T159 (triangular truncation of 159), N80 (80 latitude circles, pole to equator), L60 (model levels), 23 pressure levels and 15 isentropic levels.
ERA-15 has a resolution of T106 with 31 vertical hybrid levels.

For a list spectral, gaussian and eqivalent lat/lon grids see the Open IFS FAQ: What does the 'T' mean in 'T511', 'T1279' etc? How do I know the grid from from the 'T' number?

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