positions

The source time each tile shows.

A tile is sampled at its leading edge rather than its centre, which is what makes a zoom step a superset of the one before it: doubling the scale turns tile i into tile 2i and asks only for the times that fall between.

Each entry is computed when it is read rather than when the grid is built, so a grid over a long source at a close zoom holds nothing until something asks it for a position.