On 11/30/2015 4:38 PM, Matthijs Melissen wrote:
* Improved admin boundaries and labels on low zoom levels
This might be of interest to maintainers of other stylesheets, so I
thought I'd expand on the method used in
https://github.com/gravitystorm/openstreetmap-carto/pull/1938
admin_level=4 boundaries in large admin_level=2 countries are shown,
based on a SQL query that sees what the areas are in. The result is that
US states, Canadian provinces, and a few other large subdivisions are
rendered sooner, which is a fairly common way of rendering z3-z5.
The osm-carto method is not what I would recommend for most styles, and
not what I would do when designing a typical stylesheet.
Two constraints for osm-carto are
- We want to minimize preprocessed data because it interferes with the
update cycle
- We want to avoid the use of any non-OSM data
- We want to avoid special-casing countries
The technical result is that the layer has to come from the osm2pgsql
rendering tables, which drives the use of SQL which checks the size of
the containing country.
Typically, I would recommend
- Using preprocessed boundaries of some sort at this zoom, either
Natural Earth or one of the OSM-based sets
- Special casing by country code or a similar property, showing
admin_level 4 boundaries specifically for the US, Canada, and other
similar countries.
This is likely to be simpler and easier to develop. The performance of
the osm-carto method is pretty good, constrained mainly by the large
geometries involved.
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