Major road minzoom

[wipfli]

In many development countries the fastest roads are of type major_road. However, such roads show only up starting from z6 and more. Motorways would usually be shown at z3 to z6 but if a country has no motorways, the map looks very empty at these zoom levels.

Example: Tanzania at z4.5 looks like this:

While Switzerland at z4.5 looks like this:

[nvkelso]

A few solutions (there are probably more):

• Brute force: Show non-motorway roads earlier, at expense of clutter in more developed countries and large file size increases in same tile areas.
• Algo: Calculate directions between major cities shown at the next zoom (or zoom +2) using that "next" zoom's place nodes and OSM road network and show the deduplicated way results (to reduce tile size) at the current zoom, regardless of the highway class, while preserving the highway tag so motorways versus non-motorway could optionally be styled distinctly. This should ideally be done per data release but in practice happens one-off and then can go stale with bit rot and operational decay.
• Curation: Switch to Natural Earth for low zoom roads from OSM. While this would provide coverage and not be too big a tile size incrase, downside is the roads there in NE v5 are out of date and don't match up geometry wise on zoom in to OSM zooms. However, NE v6 is around 50% done redrawing the roads to solve that problem. Overtime the curation would need to be updated.

[wipfli]

What about a per-country minzoom? For example in Switzerland I am happy with trunk at z6, but in Tanzania I would prefer trunk already at z3. This of course leads to some questions about styling...

[nvkelso]

What about a per-country minzoom? For example in Switzerland I am happy with trunk at z6, but in Tanzania I would prefer trunk already at z3. This of course leads to some questions about styling...

That can work well for clusters of countries that all have the same network pattern (eg trunk versus motorway more common as highest functional class in the country). It works less well if a more developed country is next to a less developed country and the roads might appear to start/stop at country boundaries because different threshold were used. In Europe this could be observed between Germany and Poland and Poland and Ukraine. In Africa this might add extra road detail in say Egypt and South Africa but overall improve the whole continent (a collection of countries with a localized config enabled).

It could be a good interim approach, but is blocked here in Protomaps basemap repo by #44. Now that the NL.net grant is active this may be a priority @bdon?