3.10. `smoc`: HEALPix Multi-Order Coverage map
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## 3.10. `smoc`: HEALPix Multi-Order Coverage map #

The datatype smoc allows one to define and manipulate any region of the sky in such a way that basic operations like union, intersection can be performed very efficiently. It is dedicated to VO applications or VO data servers for building efficient procedures for which mapping knowledge is required like generic catalog cross-match, computation of data set intersections, etc. This encoding method is called "Multi-Order Coverage map" or "MOC".

The goal behind the MOC is to get a spherical geometry description for any sky region in order to provide very fast and accurate union, intersection and equality operations between them. In order to accomplish this task, we based the system on a regular and hierarchical partitioning of the sphere, a process called tessellation, yielding pixels of different size. At level 0, the sky is divided into 12 pixels numbered 0 to 11. At the maximally defined level 29, there are 12 * 4^29 = 3458764513820540928 pixels.

The MOC standard is described at http://www.ivoa.net/documents/MOC/

    level/pixel1-pixel2 [...] level/pixel1-pixel2 [...]

Example 3.14. The empty set

    sql> SELECT smoc '0/';

Example 3.15. The whole sky

    sql> SELECT smoc '0/0-11';

Example 3.16. A disk around the point (0, 0) with radius 1 at MOC level 2

    sql> SELECT smoc '0/4 1/0 2 12-13 34-35 45 47 2/4 6 12 14 56-57 60-61 88 90-91 116-117 119 130-131 134-135 177 179 185 187';

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