The skymap package provides tools for creating pixelizations of the sky to define tracts and patches for imaging data.
A sky map describes a pixelization of image data that covers most or all of the sky.
The imaging data is arranged as a sequence of overlapping rectangular “tracts”.
Each tract is, in essence, a single large exposure.
However, tracts are typically too large to fit into memory, so tracts are subdivided into rectangular, possibly overlapping “patches”.
The patch size is chosen to easily fit into memory.