Tasmania low-zoom road network
australia-oceania/australia/tasmania · 2026-10-15

Pipeline

• Logical edge graph — the OSM extract's road ways are merged into logical edges at junctions, keeping each edge's functional class (motorway → trunk → primary → secondary → tertiary → unclassified).
• City anchors — place nodes carrying population are scanned from the same extract and thresholded per zoom (500k at z2 down to 5k at z7).
• Structural Betweenness Sampling — origin/destination pairs are sampled between anchors and each is routed through a GraphHopper graph built from this same extract. Every logical edge a route rides gets a vote, so importance is measured by the traffic a road would actually carry between populated places, not by its tag.
• Bypass and ring detection — edges that skirt a settlement faster than the route through it are flagged, so a beltway survives when the high street does not.
• Per-cell budgets — the region is tiled into h3 resolution-7 hexagons (~5.2 km²) and each cell gets a road-length budget per zoom, so empty country keeps its one road through and a dense city does not swallow the whole quota.
• Selection — a greedy pass spends each budget on the highest-scoring edges (betweenness + functional class), subject to a class floor per zoom: motorway only at z2, trunk from z3, primary z4, secondary z5, tertiary z6. The motorway/trunk skeleton is always taken whole and does not charge the budget; a backbone repair pass reconnects anything the budget cut.
• Monotonic reveal — each zoom is a superset of the one below, so nothing ever disappears as you zoom in.
• Vector tiles — one PMTiles archive per zoom band with its own minimum zoom, streamed over HTTP Range. The reveal follows the map because the lower classes genuinely do not exist at continental scale.

Result

• 11,758 logical edges selected across zooms 2-7
• 18 MB of streamed vector tiles, six zoom bands
• 10 cities routes were sampled between, drawn as dots coloured by the zoom they first anchored at — click one for its name and population
• Each zoom band splits into routed (roads sampled routes rode at that zoom) and name/kind (admitted by class or name: the motorway/trunk skeleton, class score, backbone repair, rural top-up, or the rest of a routed corridor). Density rises one step per zoom: a band's routed roads appear at its zoom, its name/kind roads one zoom later. Car ferries are drawn in blue.

Not done in this run

• Bypass and ring flags are computed and exported but not styled differently.
• Layers are cumulative, so a higher zoom's tiles repeat the roads below it.