[OSM-newbies] How does Mapnik know which nodes are "important"?

[Kelly Jones]

This OSM map:

http://tile.openstreetmap.org/4/2/6.png

displays San Francisco and Los Angeles, but not the smaller city of
San Diego, even though San Diego is in the clip area.

How does Mapnik know which nodes are more important than others? In
other words, how does Mapnik decide which nodes to render?

If OSM could impose an "importance ordering" on all nodes, we might be
able to generate maps very efficiently in real-time using QuadTiles.

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