Add optional height and min_height to the buildings layer

[MichaelKreil]

The buildings layer currently carries only dummy=1. I'd like to propose adding optional height and min_height fields so that Shortbread tiles can drive 3D extrusion.

We've implemented this in the VersaTiles Planetiler Shortbread profile and it works well in production:

• Live demo (3D buildings from Shortbread tiles): https://datajournal.org/eclipse-2026/en/
• Reference implementation: https://github.com/versatiles-org/planetiler/blob/feature/shortbread-java-profile/planetiler-shortbread/src/main/java/com/onthegomap/planetiler/shortbread/layers/Buildings.java

Proposal

Add to the buildings layer (polygon, z14):

Field Name	Type	Description
height	number	height of the building in meters, optional
min_height	number	height of the bottom of the building in meters, optional

Both fields are optional: producers may omit them entirely (a strict 1.0-style implementation stays conformant), and consumers that don't render 3D simply ignore them. This is purely additive - geometry, zoom range and existing attributes are unchanged.

The naming matches the other general-purpose OSM schemas, so styles and tooling carry over: OpenMapTiles' building layer has render_height/render_min_height, and the Protomaps basemap has exactly height and min_height as numbers.

Derivation

Same rule as OpenMapTiles and Protomaps, which makes it familiar to most implementers:

• height:

  1. height=* or building:height=*, parsed as meters (accepting the usual unit suffixes), else
  2. building:levels=* (or levels=*) × 3.66 m, else
  3. omitted - see below.

• min_height:

  1. min_height=* or building:min_height=*, else
  2. building:min_level=* (or min_level=*) × 3.66 m.

  Emitted only when min_height > 0.

• Implausible values (≥ 3660 m, i.e. 1000 levels - almost always tagging errors) are dropped.

No synthetic default. A building with no height information at all should simply have no height - absent means "unknown". A style that wants every building extruded can supply its own fallback with a single MapLibre coalesce. This keeps tiles smaller, since the majority of OSM buildings have no height tagging.

Open questions

• Rounding / type. We round height up and min_height down to whole meters to keep tiles small; the spec could either mandate integers or leave the precision to the implementation.
• Relation to buildings layer containing dummy=1 #77. This is the strongest argument for dropping dummy=1: the reason it exists is that the layer has no properties, and height gives the layer a genuinely useful one. The two are independent though - dummy can be dropped with or without this.
• Follow-up, not part of this issue: OSM Simple 3D Buildings building:part polygons plus a way to mark the relation outline that must not be extruded (OpenMapTiles uses a hide_3d flag; Protomaps distinguishes kind=building from kind=building_part), and later possibly roof:shape / roof:height / colour. We have building:part implemented as a separate opt-in as well, but it's a bigger discussion and I'd rather see plain height land first.

Happy to write the spec text and example fixtures for 1.1 if there's agreement in principle.

[pnorman]

I'm not sure on the height overall, but I don't why it would be optional. That would break compatibility between shortbread tilesets.