Took a nice loop around prospect park, this time K came with too. Played around with someone else's rtree code for selectively rendering the parts of a map outline that are actually visible on the screen. Still sort of unclear to me how exactly people handle rendering of the ocean and other large-outline yet filled objects in practice. Nosing around the Open Street Map wiki and source I see that they do indeed keep the source-of-truth data as a bunch of disparate ways each tagged natural:coastline, and then it's a subsequent validation pass to make sure that they all add up to a bunch of closed curves. I guess when you rasterize one tile you just kind of add more cycles around the tile boundary, since you have a handedness convention about which side of the curve the land is on?