HEALPix

HEALPix (sometimes written as Healpix), an acronym for Hierarchical Equal Area isoLatitude Pixelisation of a 2-sphere, can refer to either an algorithm for pixelisation of the 2-sphere, an associated software package, [http://healpix.jpl.nasa.gov/ HEALPix software site] at JPL] or an associated class of map projections [http://www.atnf.csiro.au/people/mcalabre/WCS/mhg_20070626.pdf Mapping on the HEALPix grid] , Calabretta, Roukema, 2007] .

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 (pixelising) the resulting region of the 2-plane. In particular, when one of these projections (the H=4, K=3 HEALPix projection), is followed by a pixelisation of the 2-plane, this is generally referred 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 (diamonds) on the plane followed by the binary division of these facets into pixels, though it can be derived without using the projection. [http://arxiv.org/abs/astro-ph/9812350 Analysis Issues for Large CMB Data Sets] , K.M. Gorski, E. Hivon, B.D. Wandelt, 1998] . The associated software package HEALPix implements the algorithm. The HEALPix projection (as a general class of spherical projections) 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 (WCS) by the IAU FITS Working Group on April 26, 2006.cite web
last =Pence
first = William D.
authorlink =
coauthors =
title =FITS World Coordinate System (WCS)
work =
publisher = High Energy Astrophysics Science Archive Research Center (HEASARC)
date =
url = http://fits.gsfc.nasa.gov/fits_wcs.html
format =
doi =
accessdate = 2007-01-09 ]

The spherical projection combines a cylindrical equal area projection for the equatorial regions of the sphere and a pseudocylindrical equal area projection, the Collignon projection, for the polar regions.

As the name indicates, at a given level in the hierarchy the pixels are of equal area (which is done by bisecting the square in the case of the H=4, K=3 projection) 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 (which can be inversely projected back to quadrilaterals with non-geodesic sides on the 2-sphere) and every vertex joins four pixels, with the exception of eight vertices which each join only three pixels.

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 (HTM)cite web
last =
first =
authorlink =
coauthors =
title = SkyServer.org - HTM: Hierarchical Triangular Mesh
work =
publisher =SkyServer
date =June 6, 2006
url =http://skyserver.org/htm/
format =
doi =
accessdate =2007-02-05 ] cite web
last =Szalay
first =Alex
authorlink =
coauthors =Jim Gray, Gyorgy Fekete, Peter Kunszt, Peter Kukol, Ani Thakar
title =Indexing the Sphere with the Hierarchical Triangular Mesh
work =
publisher = Microsoft
month =September | year =2005
url =http://research.microsoft.com/research/pubs/view.aspx?tr_id=977
format =
doi =
accessdate =2007-02-05 ] . 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 great and small circles.

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 [http://space.mit.edu/home/tegmark/icosahedron.html What is the best way to pixelize a sphere?] ] .

References

External links

* [http://www.progonos.com/furuti/MapProj/Normal/ProjPCyl/ProjPCH/ discussion and HEALPix maps] of the Earth