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HEALPix is an acronym for Hierarchical
Equal Area
isoLatitude Pixelization
of a sphere. As suggested in the name, this pixelization produces a
subdivision of a spherical surface in which each pixel covers the same
surface area as every other pixel. The figure below shows the partitioning
of a sphere at progressively higher resolutions, from left to right. The
green sphere represents the lowest resolution possible with the HEALPix
base partitioning of the sphere surface into 12 equal sized pixels. The
yellow sphere has a HEALPix grid of 48 pixels, the red sphere
has 192 pixels, and the blue sphere has a grid of 768 pixels
(~7.3 degree resolution).
Another property of the HEALPix grid is that the pixel centers, represented
by the black dots, occur on a discrete number of rings of constant latitude,
the number of constant-latitude rings is dependent on the resolution of the
HEALPix grid. For the green, yellow, red, and blue spheres shown, there are
3, 7, 15, and 31 constant-latitude rings, respectively.
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Two high-resolution examples of applications of HEALPix are shown below.
The first example (left) is an Earth topography map composed of 3,145,728 pixels (~7 arcmin
resolution) and the second (right) is a model of the Cosmic Microwave Background (CMB)
radiation temperature anisotropy, composed of 12,582,912 pixels
(~3.4 arcmin resolution).
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