## Grid geometry

ERA5 data is produced and archived as spectral coefficients or on a reduced Gaussian grid, which has quasi-uniform spacing over the globe. Reduced Gaussian grids have a series of evenly spaced data points along each parallel (latitude), and parallels spaced at quasi-regular intervals. Near the poles there are only a few points along a parallel, but close to the equator there are many more data points along a parallel.

A reduced Gaussian grid is quite different from a Cartesian grid (a two-dimensional rectangular grid).

The ERA5 HRES (High Resolution) and EDA (Ensemble) data are archived in these grid representations:

- Some (upper air) parameters are archived as spectral coefficients (indicated in the data as 'sh', Spherical Harmonics) with a triangular truncation of T639 (HRES) and T319 (EDA)
- Some (including all the surface) parameters are archived on a reduced Gaussian grid with a resolution of N320 (HRES) and N160 (EDA). These grids are so called "linear grids", sometimes referred to as TL639 (HRES) and TL319 (EDA). N320, for example, refers to the number of latitude circles from pole to equator.

For wave data, both HRES and EDA are archived on a reduced latitude/longitude grid.

For a list of spectral, Gaussian and equivalent lat/lon grids see the Open IFS FAQ, section "OpenIFS questions: general and runtime", and there "What does the 'T' mean in 'T511', 'T1279' etc?" and "How do I know the grid from the 'T' number?"

When you download ERA5 data, the grid geometry of your output data depends on the selected data format:

- If you request the data in the native GRIB format it is delivered with the above grid geometry.
- If you request the data in NetCDF format, it is automatically converted and interpolated from the above grids to a regular lat/lon grid.

## Coordinate system

All gridded data is made available in Decimal Degrees, lat/lon, with latitude values in the range [-90;+90] referenced to the equator and longitude values in the range [0;360] referenced to the Greenwich Prime Meridian. Some software applications automatically display these longitudes in the range [-180;+180].

## Interpolation

When you download ERA5 data you have the option to interpolate the data to a custom grid and horizontal resolution (eg. 'grid':'0.5/0.5'). 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 T639 is 0.28125 deg (360/(2*(639+1))). However, the GRIB1 format only supports three decimals, so we recommend that you round the resolution to 0.25 deg (giving 360/0.25=1440 equally-spaced grid points). Specifying a higher resolution is technically possible, e.g. as 'grid':'0.1/0.1', but this only oversamples the data and does not improve the accuracy of the data. Specifying a coarser resolution is also possible, but in this case, care should be taken to avoid aliasing.

If you request data in **NetCDF** format ('format':'netcdf'), interpolation to a regular grid is mandatory, because ECMWF's NetCDF implementation only supports regular grids. Specify the desired lat/lon grid with the 'grid' keyword, for example 'grid':'0.25/0.25'.

## Visualisation of regular lat/lon data

If you use ERA5 data in a regular lat/lon grid, think of the data as point values, with a regular spacing, as shown in (a): here global ERA5 data was interpolated to a regular lat/lon grid with resolution r = 0.25 deg. The 'top left' ERA5 data point is always at Longitude=0 ; Latitude=90, with further grid points spaced by r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90. Many software applications by default visualise regularly spaced data as a continuous tiled surface, as in (b). If you use this visualisation, think of coordinates as referencing the centroids of the tiles. However, the ECMWF interpolation software does not conserve area integrals, so this visualisation can be misleading. We recommend you conceptualise ERA5 data as in (a). | (a) Visualisation of regular lat/lon data as point matrix (b) Visualisation of regular lat/lon data as a continuous tiled surface |

## 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 data if possible. This GRIB1 sphere does not have an EPSG code.

## Comparison with other ERA products

ERA-Interim is produced and stored as spectral coefficients with a truncation of T255 or on the N128 reduced Gaussian grid (depending on the parameter). See ERA-Interim: What is the spatial reference

- ERA-40 has a resolution of T159, N80.
- ERA-15 has a resolution of T106, N80.