Flattening

Summary

The flattening of an ellipse or of an ellipsoid of revolution measures how far it is squashed compared with a circle or a sphere. It is the number $f = (a - b)/a$, where $a$ is the semi-major axis and $b$ the semi-minor axis. In geodesy it is the standard shape parameter of a reference ellipsoid, together with $a$. For the Earth, $f \approx 1/298.257$.