Network clean-up

OSM data has small gaps: a footway that ends a metre short of the road it meets, a road cut in two by the area's border. Two steps deal with them, then an optional check.

Step	Modes	Runs when
Connect dangling paths	walking, cycling	clip.connectivity_rescue (on), in a build from a PBF
Component filter	all	a boundary is set, and clip.keep_largest_component (on) or clip.min_component_edges > 1
Validation	all	validation.enabled (off; on in the config template)

Connect dangling paths

A dangling end is a node that touches only one edge, where that edge is a cycleway, footway, path, steps, pedestrian or track. For each one, the build adds a straight connector edge to the nearest node within clip.connect_snap_m metres (default 10), in both directions. Without it, the path would be a separate piece and the component filter would drop it.

A connector edge has:

• a negative osm_id, -min(a, b): a and b are the osm_ids of the path and of the road at the node it joins. Select connectors with osm_id < 0.
• the highway of the path and oneway = FALSE; no name, lanes or other tags (NULL). In cycling, a connector that extends a dismount path is dismount too.
• an edge_id from the usual formula.

Monaco:

Mode	Dangling ends joined	Connector edges added	Left after the component filter
walking	254	508	490
cycling	512	1,024	986

Limits: it joins the nearest node, not the nearest point of an edge, so a connector can be longer than the real gap. The distance ignores levels, so it can join a path to a road on a bridge above it. An end with no node within the distance is not joined. Driving isn't repaired.

Component filter

The filter keeps the largest connected piece of each network and drops the rest: stubs cut off by the border and small isolated pieces. It counts pieces by edges, and ignores direction (weakly connected). Dropped edges also leave edge_graph, turn_restrictions, nodes and edge_id_map. It works on the routable edges only: private_edges are not filtered.

Key	Default	Does
clip.keep_largest_component	true	keep only the largest piece
clip.min_component_edges	1	also keep every other piece with at least this many edges

It runs only when a boundary is set (a whole country has real islands) and, for a PBF build, when the edge graph is built. It runs in clip builds too. Monaco:

Mode	Edges dropped	Pieces dropped
driving	79	15
walking	268	51
cycling	225	35

Because direction is ignored, the kept network can still hold a one-way dead end: an edge you can reach but not leave. Monaco, driving: 25 such edges.

Where the border cuts

Build from a PBF. The PBF is cut with osmium (strategy smart by default, complete_ways when options.build_features: false; options.clip_strategy sets it): every way with a node inside the boundary is kept whole, so roads that cross the border stick out past it. Then the component filter removes what doesn't connect. boundary.buffer_m grows the boundary first.

Clip from a parent build. clip.predicate decides which edges are copied:

clip.predicate	Keeps an edge when
intersects (default)	it touches the boundary: roads that cross the border are kept whole
within	it lies completely inside: no stubs, but roads that cross the border are lost
centroid	its middle point is inside

duckosm extract always uses intersects.

Validation

With validation.enabled, each mode is checked once it is built. A failed check stops the build unless validation.fail_on_error: false. How to switch it on and what the output looks like: Check a build.

Check (validation. key)	Passes when
assert_single_component	the largest piece holds at least 99.9% of the edges
assert_no_stranded_named	no edge with a name is outside the largest piece
assert_unique_node_id	no node_id appears twice in nodes
assert_way_length_conserved	no stretch of a kept OSM way is missing while an edge of the same way joins its two ends
warn_layer_without_structure	a warning only: edges with layer ≠ 0 but no bridge or tunnel tag

The first two run only when the component filter ran. assert_way_length_conserved is skipped in a clip build, which has no ways table.