  The natural spherical projection associated with the Hierarchical Equal Area
and isoLatitude Pixelisation, HEALPix, is described and shown to be one of an
infinite class not previously documented in the cartographic literature.
Projection equations are derived for the class in general and it is shown that
the HEALPix projection suggests a simple method (a) of storing, and (b)
visualising data sampled on the grid of the HEALPix pixelisation, and also
suggests an extension of the pixelisation that is better suited for these
purposes. Potentially useful properties of other members of the class are
described. Finally, the formalism is defined for representing any member of the
class in the FITS data format.
