path.projection()

path.projection([projection])

If a projection is specified, sets the current projection to the specified projection. If projection is not specified, returns the current projection, which defaults to null. The null projection represents the identity transformation: the input geometry is not projected and is instead rendered directly in raw coordinates. This can be useful for fast rendering of pre-projected geometry, or for fast rendering of the equirectangular projection.

The given projection is typically one of D3’s built-in geographic projections; however, any object that exposes a projection.stream function can be used, enabling the use of custom projections. See D3’s transforms for more examples of arbitrary geometric transformations.