Fragmented line paths at low zoom levels

[zhibek]

Sorry for this basic question - I'm new to vector-based web mapping. If this issue is due to something outside of Planetiler, feel free to tell me!

I'm converting specific OSM ways to line paths in a very simple Planetiler schema - its creating a single layer for "main" railways, setting a minimum zoom level of 4 (see schema below). After generating the mbtiles output, I'm checking it in various tools (tileserver-gl example shown below). At low zoom levels, the line paths appear fragmented (i.e. they contain gaps).

Given this issue is displayed regardless of which application I use to view the mbtiles, I don't believe the problem is due to the method of display. I've checked the OSM source and I can't see anything obviously wrong with the ways in the locations where there are gaps. Therefore my basic conclusion is there's either:
(a) An issue with how the mbtiles are being generated (possibly the schema definition I've used needs extra attributes to avoid this problem?).
or;
(b) Fragmented line paths are intrinsic to vector tiles and map styles must be crafted to obscure this problem.

I'd appreciate help in understanding if this issue is related to Planetiler (or the schema I'm using) or if its a more general issue with vector tiles.

For reference, the use-case I'm investigating is generating tiles to be used for displaying railways across large areas (displaying these line strings effectively at zoom levels 6~8 is important for this use-case).

Example showing fragmented line paths, viewed in tileserver-gl

Example schema

schema_name: Railway Map
schema_description: Railway Map
attribution: <a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>
sources:
  osm:
    type: osm
    url: geofabrik:greater-london
layers:
- id: railway_line_main
  features:
  - source: osm
    geometry: line
    min_zoom: 4
    include_when:
      __all__:
      - railway: rail
      - usage: main
    exclude_when:
      service: __any__

[bdon]

The issue is (a) above. The YAML config method does not perform any line merging in tile-post processing, that issue is tracked here.

Until that feature is added, your best option is to write a Java profile that explicitly calls LineMerge functions with parameters tuned to your feature geometries and tags.

Also, tippecanoe is an alternative tool with a greater focus on automated generalization at low zooms, though it's not specific to OSM inputs.

[zhibek]

Thank you for explaining @bdon. That's clear to me now.

I believe an example of a profile that merges in this way is BikeRouteOverlay:

planetiler/planetiler-examples/src/main/java/com/onthegomap/planetiler/examples/BikeRouteOverlay.java

Lines 122 to 126 in a4ea44f

	return FeatureMerge.mergeLineStrings(items,
	0.5, // after merging, remove lines that are still less than 0.5px long
	0.1, // simplify output linestrings using a 0.1px tolerance
	4 // remove any detail more than 4px outside the tile boundary
	);

[zhibek]

From experimenting, I think minimum size is also an important aspect of why I'm seeing gaps in lines at low zooms:
#349

(I'd prefer to stick with YAML based schema if possible in preference to maintaining a Java profile)