1000s of gardens in Edinburgh zoom 12 tile

[nvkelso]

• Fix min_zoom calculation for garden features
• Followup about requiring (or not requiring) name is in Require that most POIs have names #1186.

While investigating mapsforge/vtm#341 it looks 1000s of features in the pois layer of kind:garden that don't have names, creating 18,000+ features in some tiles like:

• http://tile.mapzen.com/mapzen/vector/v1/all/12/2011/1276.topojson

Browsing around OpenStreetMap data here most backyards have been tagged as unnamed, private gardens, example:

• http://www.openstreetmap.org/way/273274870

There appears to be an error in the vector tile filter code where gardens are all getting min_zoom 12 instead of throttling that by area (and really we should require some value of name – should we in general no, but for what kinds of features, feels like separate issue):

• https://github.com/tilezen/vector-datasource/blob/master/yaml/pois.yaml#L441-L444

Specifically:

 min_zoom: { min: [ 12, { max: [ { lit: zoom }, { lit: *tier6_min_zoom } ] }, 16 ] }

Should probably drop the 12, for:

 min_zoom: { min: [ { max: [ { lit: zoom }, { lit: *tier6_min_zoom } ] }, 16 ] }

Where tier6_min_zoom already is:

  - &tier6_min_zoom |
      CASE WHEN way_area >= 500000 THEN 12
           WHEN way_area >= 100000 THEN 13
           WHEN way_area >=  50000 THEN 14
           WHEN way_area >=   5000 THEN 15
           ELSE                         16
      END

I don't see any other filters at first glance that have this error, but let's take another pass thru as part of this issue.

[nvkelso]

@zerebubuth I'm still seeing lots of small gardens in the example zoom 12 tile, like this one I expected at zoom 15:

{"type":"Point",
"properties":
{"source":"openstreetmap.org",
"kind":"garden",
"name":"Moray Place",
"area":44071,"tier":6,"min_zoom":12.0,"id":23187947},
"coordinates":[2020,2508]
}

Maybe a bad min_zoom database migration?

[nvkelso]

This is working :)