HEALPix, an acronym for Hierarchical Equal Area isoLatitude Pixelisation of a 2-sphere, refers to either an algorithm for pixelisation of the 2-sphere or to the associated class of map projections. The pixelisation algorithm was devised in 1997 by Krzysztof M. Górski at the Theoretical Astrophysics Center in Copenhagen, Denmark, and first published as a preprint in 1998.
Projection and pixelisation
The HEALPix projection is a general class of spherical projections, sharing several key properties, which map the 2-sphere to the Euclidean plane. Any of these can be followed by partitioning the resulting region of the 2-plane. In particular, when one of these projections is followed by a pixelisation of the 2-plane, the result is generally known as the HEALPix pixelisation, which is widely used in physical cosmology for maps of the cosmic microwave background. This pixelisation can be thought of as mapping the sphere to twelve square facets on the plane followed by the binary division of these facets into pixels, though it can be derived without using the projection. The associated software package HEALPix implements the algorithm. The HEALPix projection is represented by the keyword HPX in the FITS standard for writing astronomical data files. It was approved as part of the official FITS World Coordinate System by the IAU FITS Working Group on April 26, 2006. The spherical projection combines a cylindricalequal area projection, the Lambert cylindrical equal-area projection, for the equatorial regions of the sphere and a pseudocylindricalequal area projection, an interrupted Collignon projection, for the polar regions. As the name indicates, at a given level in the hierarchy the pixels are of equal area and their centers lie on a discrete number of circles of latitude, with equal spacing on each circle. The scheme has a number of mathematical properties which make it efficient for certain computations, e.g. spherical harmonic transforms. In the case of the H=4, K=3 projection, the pixels are squares in the plane and every vertex joins four pixels, with the exception of eight vertices which each join only three pixels.
Usage and alternatives
The pixelisation related to the H=4, K=3 projection has become widely used in cosmology for storing and manipulating maps of the cosmic microwave background. An alternative hierarchical grid is the Hierarchical Triangular Mesh. The pixels at a given level in the hierarchy are of similar but not identical size. The scheme is good at representing complex shapes because the boundaries are all segments of circles of the sphere. Another alternative hierarchical grid is the Quadrilateralized Spherical Cube. The 12 "base resolution pixels" of H=4, K=3 HEALPix projection may be thought of as the facets of a rhombic dodecahedron. The H=6 HEALPix has similarities to another alternative grid based on the icosahedron.
