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LSST Data Management Base Package
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The Hierarchical Triangular Mesh (HTM) is described in the work of A. Szalay, T. Budavari, G. Fekete at The Johns Hopkins University, and Jim Gray, Microsoft Research. See in particular the following paper:
"Indexing the Sphere with the Hierarchical Triangular Mesh" Szalay, Alexander S.; Gray, Jim; Fekete, George; Kunszt, Peter Z.; Kukol, Peter; Thakar, Ani 2007, Arxiv e-prints
http://arxiv.org/abs/cs/0701164 http://adsabs.harvard.edu/abs/2007cs........1164S
To summarize very briefly: HTM partitions the unit sphere into 8 root triangles by splitting it with the planes x = 0, y = 0, and z = 0. A triangle is subdivided into 4 children by connecting the midpoints of its edges with geodesics, and each root triangle is recursively subdivided to a fixed subdivision level to obtain a pixelization of the sphere. The root triangles are assigned indexes 8-15, and the children of a triangle with index I are assigned indexes [4*I, 4*I + 4).
Budavári, Tamás; Szalay, Alexander S.; Fekete, György "Searchable Sky Coverage of Astronomical Observations: Footprints and Exposures" Publications of the Astronomical Society of Pacific, Volume 122, Issue 897, pp. 1375-1388 (2010).