Remove early water labels

[nvkelso]

Picking up from where #1477 leftoff... There are still a lot of water labels in the mid-zoom tiles.

This is controlled with this config:

• https://github.com/tilezen/vector-datasource/blob/master/queries.yaml#L805-L837

Edit something like (adjusting low zooms and then moving others by a zoom to filter out along the way each zoom):

  # some water polygon features are too small to need a label - and if we label
  # them then the label is often so prominent that it hides the polygon it's
  # for! see the images here for examples of this:
  # https://github.com/tilezen/vector-datasource/issues/1477#issuecomment-447162484
  #
  # we want to reset min_zoom on these features and screen them out if they
  # don't meet minimum sizes per zoom.
  - fn: vectordatasource.transform.update_min_zoom
    params:
      source_layer: water
      start_zoom: 4
      end_zoom: 16
      # the inclusion of "None" in the list might seem odd, but None compares
      # numerically smaller than any integer, so acts as a default for this
      # table.
      min_zoom: >-
        [min_zoom for min_zoom, area_threshold in [
            (4,  120000000000),
            (5,   80000000000),
            (6,   40000000000),
            (7,   10000000000),
            (8,     500000000),
            (9,     200000000),
            (10,     50000000),
            (11,     10000000),
            (12,      1000000),
            (13,       500000),
            (14,        50000),
            (15,        10000),
            (16,         None),
        ] if area >= area_threshold][0]
      where: >-
        kind == 'lake' and label_placement

Then there are also removing line labels, which is controlled around, it should be done before label placements are generated:

• https://github.com/tilezen/vector-datasource/blob/master/queries.yaml#L508-L515

For the following kinds:

• kind: sea, zoom < 4
• kind: lake, zoom < 5
• kind: river, zoom < 12
• kind: [canal], zoom < 13
• kind: [dam,stream], zoom < 14
• kind: [ditch,drain], zoom < 15
• is_tunnel: true, zoom < 16

Something like:

  - fn: vectordatasource.transform.drop_properties
    params:
      source_layer: water
      start_zoom: 0
      end_zoom: 16
      # short-hand for "name" property in the list below means all name-like
      # properties.
      all_name_variants: true
      properties:
        - name
      where: >-
        (kind == 'sea' and zoom < 4) or
        (kind == 'lake' and zoom < 5) or
        (kind == 'river' and zoom <  12) or
        (kind == 'canal' and zoom <  13) or
        (kind == 'dam' and zoom <  14) or
        (kind == 'stream' and zoom <  14) or
        (kind == 'ditch' and zoom <  15) or
        (kind == 'drain' and zoom <  15) or
        (is_tunnel == true and zoom <  16)

[nvkelso]

We also want to fix the min_zoom of huge water features that pop in at zoom 8 from OSM, so they have more consistent min_zoom with what's in NE in previous zooms. One would think the transform in the first comment would fix that, but it doesn't seem to be working. Instead the basic areas > min_zoom from the polygons are carried forward to the label placement points. Both should be adjusted, though ideally the label placement ones also get applied.

• https://github.com/tilezen/vector-datasource/blob/master/yaml/water.yaml#L23-L40

The min_zoom on OSM features is set there, and currently has a zoom 7 area threshold, but we need to extend that.

The OSM data itself only comes in at zoom 8, per:

• vector-datasource/queries.yaml

  Lines 129 to 135 in 1f4b5d9

  	osmdata:
  	- template: osmdata-earth.jinja2
  	start_zoom: 8
  	- template: osmdata-water.jinja2
  	start_zoom: 8

So it's okay to have smaller min_zooms in the data, it won't actually make those OSM features show up earlier.

[nvkelso]

To promote more feature merging, we'll also want to drop names and many other properties from the water polygons (and sometimes lines) at low and mid-zooms before they are merged.

[nvkelso]

Area filter something like:

zoom	area
1	20,000,000,000
2	10,000,000,000
3	5,000,000,000
4	2,000,000,000
5	500,000,000
6	200,000,000
7	50,000,000

[nvkelso]

PR is #2010.

[nvkelso]

In #2010 because of:

    (kind == 'water' and zoom <  12) or

and the way OSM tags water and Tilezen ETLs the tags, this results in too many water labels being removed.

Related: if #984 were implemented then this problem would partly resolve itself. But we should consider a lower zoom number for water, too.

This kind issue is listed as a gotcha in https://github.com/tilezen/vector-datasource/blob/5dfc1d1d697a648f846646a4a20596b2b98b19e6/docs/layers.md#water as a planned fix. There's a debate to be had if that would be a breaking change (even if planned since v1.0), so let's adjust the water zoom here only, then to #984 in the v2 milestone.

[nvkelso]

Via #2047