1. A method performed by one or more computers, the method comprising: receiving a plurality of input tiles of first imagery, the first imagery being in a first projection; generating, from each of the plurality of input tiles of first imagery, one or more respective fragments for each of one or more output resolution levels, each fragment being a re-projection of imagery from the first projection to a second projection at the respective output resolution level, including: approximating vertical stretch for each tile at each respective output resolution level, and selecting a zoom factor for counter-balancing the vertical stretch; tagging each fragment with a tag identifying an output tile and an output resolution level to which the fragment will contribute imagery; storing the fragments on multiple storage subsystems, wherein all fragments having a same tag are stored on a same storage subsystem; and assembling fragments with the same tag to produce output tiles in the second projection for each of the one or more output resolution levels.