# Spherical coordinates of ERA5 model level data

**URL:** <https://forum.ecmwf.int/t/spherical-coordinates-of-era5-model-level-data/14580>\
**Category:** C3S - Datasets and Usage\
**Tags:** ifs, era5\
**Created:** [8 January 2026 09:30 UTC](https://forum.ecmwf.int/t/spherical-coordinates-of-era5-model-level-data/14580 "2026-01-08T09:30:15Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Tobias\_Gerken](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/t/8491ac/32.png) [@Tobias\_Gerken](https://forum.ecmwf.int/u/Tobias_Gerken)\
**Post date:** [8 January 2026 09:30 UTC](https://forum.ecmwf.int/t/spherical-coordinates-of-era5-model-level-data/14580/1 "2026-01-08T09:30:15Z")

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Hello,  
I am comparing ERA5 model level data to radiosondes at specific lat-lon coordinates and have a question about the associated Earth model in the dataset.

The dataset I am working with has the following attribute for the grid:

`// global attributes:`  
`:DATA_SOURCE = “ECMWF: ERA5 GRIB 1 spherical harmonics (T639) and reduced Gaussian grid data (N320).” ;`  
`:DATA_INTERPOLATION = “CISL RDA: ERA5 GRIB 1 N320/T639 data interpolated (or synthesized) to 1280 longitude x 640 Gaussian latitude GRIB 1 data using emoslib-4.3.9 (compiled with cmake with grib_api-1.14.7 and fftw-3.3.4 support for gnu/gfortran-4.4.7).” ;`

I want to make sure that I understand the documentation ([faceconfluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation#ERA5:datadocumentation-SpatialgridSpatialGrid](http://faceconfluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation#ERA5:datadocumentation-SpatialgridSpatialGrid)) correctly.

> The IFS assumes that the underlying shape of the Earth is a perfect sphere, of radius 6371.229 km, with the surface elevation specified relative to that sphere. The geodetic latitude/longitude of the surface elevation datasets are used as if they were the spherical latitude/longitude of the IFS.
> 
> ERA5 data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the major/minor axes) and in the vertical it is referenced to the EGM96 geoid […]
> 
> For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km (note, this is inconsistent with what is actually used in the IFS), as defined in the [WMO GRIB Edition 1 specifications](https://old.wmo.int/extranet/pages/prog/www/WMOCodes/Guides/GRIB/GRIB1-Contents.html), Table 7, GDS Octet 17.
> 
> For data in GRIB2 format the earth model is a sphere with radius = 6371.2229 km (note, this is consistent with what is actually used in the IFS), as defined in the [WMO GRIB2 specifications, section 2.2.1](https://old.wmo.int/extranet/pages/prog/www/WMOCodes/Guides/GRIB/GRIB2_062006.pdf), Code Table 3.2, Code figure 6.
> 
> For data in NetCDF format (i.e. converted from the native GRIB format to NetCDF), the earth model is inherited from the GRIB data.

Based on the text above, I am wondering whether I need to convert the lat-lon coordinates given in the ERA5 model level output to WGS84 coordinates before comparing data to specific sounding locations. I understand that the ERA5 0.25 degree product is referenced with respect to WGS84, but I am not sure whether the model level dataset is modified from the original IFS grid that is spherical.

Thank you so much!

Tobias