Further places layer plastic surgery

[nvkelso]

Originally posted by @nvkelso in https://github.com/tilezen/vector-datasource/issue_comments#issuecomment-447149749

For zoom 0 we're still including too many countries. The same is true for zooms 1 and 2. Addressing this will have significant file size savings.

Seems like we're doing 512 min_zoom math instead of 256 math? Like at zoom 0 we should only see features with min_zoom < 1, but many 1.x graded features are included.

New:

Old:

`

Poland is included at zoom 2, but it's min_zoom is 3.

[nvkelso]

Probably this line can change:

https://github.com/tilezen/vector-datasource/blob/master/vectordatasource/transform.py#L8132

        if min_zoom is not None and min_zoom <= nominal_zoom + 1:

to:

       if min_zoom is not None and min_zoom <= nominal_zoom + 0.5:

Which would better match how Natural Earth is curated for min_zoom (which will remove the 1.7 features above from the zoom 1 tile, but they'll show up in zoom 2 tile and collide out min_zoom 2.0 features – good). There are some x.5 curated features in Natural Earth.

This would also force the Poland example into zoom 3 tile.

[zerebubuth]

Seems like we're doing 512 min_zoom math instead of 256 math? Like at zoom 0 we should only see features with min_zoom < 1

Our 512px tiles are designed to contain the same content as the 4 z+1 256px tiles which cover the same area. Therefore, the nominal zoom (i.e: style zoom) of a 512px tile is z+1. So the nominal zoom of the 0/0/0 512px tile is 1.

Also, because clients typically overzoom a zoom N tile for zooms <N+1, we designed the tile to contain things with a min zoom up to N+1. Therefore, the 0/0/0 tile would contain, for example, features with a min_zoom of 1.9 because the 0/0/0 tile is the one being displayed when the nominal zoom is 1.9.

I'm happy to make a change as suggested (basically dropping a whole bunch of features needed for overzooming), but we should be aware that it'll be a major change compared with how we've done things in the past. Although it might not matter, if clients aren't evaluating fractional zooms anyway.

[nvkelso]

Turns out Tangram doesn't evaluate fractional zooms. I still think it's useful to overstuff the layer a little bit, but for the places layer I think the file size savings are more important / values with x.6 and x.7 etc min_zoom would probably need much more sophisticated runtime label logic than is generally available, too?

The reason zoom 0 is still big is continent and ocean labels (in water and earth layers), adding a fix for those now to put them in zoom 1 instead (which is still MINOR version compliant change).