Programs that generate map tiles default to generating tiles for a
bounding box whose dimensions are fixed up and down the zoom stack. But
the overarchingly common use case calls this default behavior into
question. If the ultimate goal of a map is to lock down the display of a
feature at a high zoom level, then any tile outside of the inverted
pyramid whose truncated top bounds the feature at the desired zoom level
is extraneous, unnecessary.
Inspired by a game of marbles that uses a similar shape in its playing,
I call this truncated, inverted pyramid the "Tower of Prince Henry,
Reversed", and abbreviate it TOPHR.
This presentation describes modifications to TileMill, the same strategy
implemented directly through Mapnik XML, the use of the flexible mbtile
format to store the generated tiles, and presents several measures of
the resulting savings (tile generation time, number of tiles, disk
space). I'll also describe a plug in for Leaflet and an approach for
OpenLayers that ensures that map users cannot stray outside the bounds
1. It's also reminiscent of the name of a real album by The Fall or an unreal tarot card.