Depends what you want to do. If you want to compute a few very precise bearings, distances, areas, etc. (say for some navigation or surveying application) you s

Depends what you want to do. If you want to compute a few very precise bearings, distances, areas, etc. (say for some navigation or surveying application) you should use an ellipsoidal model or even something fancier if your requirements are very exacting. If you want to do computational geometry or spatial indexing with millions of objects, you might prefer to trade accuracy for speed and model with a sphere.