Wiki du Master Géographies Numériques

The planet isn't a perfect sphere but presents a bumpy aspect with numerous irregularities. “To model this surface, a more regular geometric figure is used, the ellipsoid: it is a generally spherical volume showing a flatness to the pole and used as reference for the building of map projections.”

(sources Géoconfluences, Samuel Depraz)

See : Projection (System of)