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For ERA-Interim atmospheric data the point interval on the native Gaussian grid is about 0.75 degrees. You can specify a custom grid on the data server web interface, or using the ECMWF WebAPI or using the MARS client (if you have access to it). On the web interface the default grid for ERA-Interim is lat/long, with a default resolution of 0.75x0.75 degrees (about 80km), approximating the irregular grid spacing on the native Gaussian grid.
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