The Earth is a globe and displaying it on a plane requires complex mathematical transformations known as projections. Map projections are attempts to portray the surface of the earth or a portion of the earth on a flat surface. Some distortions of conformality, distance, direction, scale, and area always result from this process. Some projections minimize distortions in some of these properties at the expense of maximizing errors in others. Some projections are attempts to only moderately distort all of these properties. Map projections convert geographic points, represented by a longitude and a latitude, to Cartesian coordinates in a planar coordinate system.

You use a particular projection depending on the area of the globe that interests you. Mercator preserves the heading and is useful for navigation. Lambert (conic) distorts distance. Lambert 1 is used for Northern France and Lambert 2 for Southern France.

The ideal characteristics for a projection would be:

The perfect projection does not exist, so you have to choose the best suited to your needs, depending on the main properties of your application and the geographic areas used in your application.