# Ellipsoids and Geodesy

This chapter delves into reference ellipsoids and their use in healpix-geo.

## Introduction

The Earth is not a perfect sphere but an ellipsoid flattened at the poles.

For more details, see:

- {doc}`../reference-system` - Complete Reference Systems
- {doc}`../tutorials/ellipsoid_basics` - Tutorial on Ellipsoids
- {doc}`../healpix/ellipsoidal-extension` - Extension of the HEALPix pixelation scheme to ellipsoidal Earth models