Add progressive administrative levels to layer "boundaries"

[superrache]

The "boundaries" layer currently supports only admin levels 2 and 4 (countries and states), displayed at zoom levels 0 and 7 respectively.
The current raster style OpenstreetMap-carto already implements progressive rendering of additional admin levels based on zoom level. Adding the same feature would significantly improve the map's usability for local navigation and context.

Proposed features:

admin_level	zoom level	description
2	0	countries
4	7	state/provinces
6	10	departements/districts
8	12	cities
9	12	city districts
10	14	neighborhoods
11	14	subdivision of neighborhoods

Complete administrative levels by country

About labeling: in OpenstreetMap-carto, boundaries are labeled along the boundary lines, on the right side of the line. I'm not sure if this feature is achievable with vector tiles and Maplibre, that might be a separate topic. Maybe in a first time, new admin_level feature could be added in "boundary_labels" layer.

[pnorman]

I thought we had another issue for this but see we don't. #140 cleaned up how this layer was defined so it should be easier to edit it now.

I'm +1 on including admin levels 6 and 8. I'm not sure where/how 3, 5, and 7 should fall into this. I'm also not sure on 9, 10, and 11.

[1ec5]

I'm not sure where/how 3, 5, and 7 should fall into this.

The odd levels are important in some countries (or some parts of those countries). They can be challenging to style, but without an odd level that’s mapped, the next level can be misleading.

Choosing the minimum zoom level for an admin level is always difficult. Countries vary so much in area, so their subdivisions do as well. Some smaller countries have hacked around this problem by skipping multiple zoom levels, but we shouldn’t assume that this completely absolves either the schema or the stylesheet from having to make decisions about, say, which countries show subdivisions at which zoom levels. For example, z7 is already quite a high zoom level at which to start showing Canadian provincial borders and labels, at least in Mercator projection. I think OpenMapTiles has erred on the side of showing dense boundaries. At least MapLibre supports decluttering labels on the client side.

About labeling: in OpenstreetMap-carto, boundaries are labeled along the boundary lines, on the right side of the line. I'm not sure if this feature is achievable with vector tiles and Maplibre, that might be a separate topic. Maybe in a first time, new admin_level feature could be added in "boundary_labels" layer.

There are two approaches to achieving boundary edge labels with vector tiles:

• Each boundary linestring feature indicates the names of the administrative areas on the left and right in each of the supported languages. This ensures the left and right edge labels line up perfectly. OpenMapTiles sort of does this, but it only indicates country codes along international borders. This forces Diplomat to implement a client-side mapping to country names using built-in browser functionality, which doesn’t necessarily match the names in OSM. I’d favor including the full set of localized names for simplicity, even if it bloats the tiles a bit.
• Each boundary is also represented by a polygon. OpenHistoricalMap does this for non-administrative boundaries such as time zone boundaries. The downsides are more bloat from redundant geometries and left and right labels that don’t quite line up.

I think boundary edge labels can be a separate feature from boundary lines. OHM split it out as tail work too, though it doesn’t need boundary edge labels quite as much because it implements #86 (comment) consistently.

[pnorman]

This is my initial thought for suitable zooms. It fixes the somewhat odd situation where we have labels for admin_level=4 coming in at z3-z5 yet the lines don't come in till z7.

Remember when comparing to something like a leaflet map that its scale to zoom relationship different than our target and is equivalent to the leaflet zooms being reduced by 1

``

admin_level value	zoom
2	0
3	2
4	3
5	4
6	7
7	9
8	11

I'm still not sure on z9, z10, and z11.