I have made a number of changes to the road rendering in my experimental map style in recent months, which i intend to discuss in a number of upcoming blog posts here. In addition to these changes i also made the decision to remove all administrative boundaries except admin_level 2 (i.e. national boundaries) from the style.

Administrative boundaries have been an odd aspect of mapping in OpenStreetMap all along due to their abstract nature. The boundary per se, as an abstract concept and as an explicit line is clearly non-verifiable according to the OpenStreetMap concept of verifiability. Still – it is often a prominent part of the geography in the minds of people, though in a different way than verifiable things. That has led to a number of rather interesting practical effects in OpenStreetMap:

• Boundaries, in particular administrative ones, but to some extent also those of nature reserves or other abstract delineations of special legal status, are the only clearly non-verifiable features in OpenStreetMap where there is clear consensus that they should be in OpenStreetMap.
• Boundaries are not only clearly non-verifiable, they are also treated as such by the OSM community. Even where the de facto boundary is formed by a physical geography feature – like a river or a mountain ridge – boundaries are deliberately not aligned to those verifiable physical features. Mappers mostly seem to treat boundaries as inherently authoritative – no matter how questionable the provenance of the boundary geometries is and sometimes even in cases where the data is evidently counterfactual (because you can see that the actual administrative reality is different).

Bottom line from the data side: Boundary data is – for the most part – neither suitable to be mapped in OSM based on the local knowledge of mappers nor is it de facto widely mapped this way. It is none the less in the OSM database (a) because – as said – it is widely part of the mental image people have of the geography and (b) because it is widely believed that it is useful to have and maintain it in a common database with the other data in OpenStreetMap (a belief that can be rightfully questioned of course).

But because boundary data is usually not part of the mapping process in OpenStreetMap, displaying it in a map meant for mapper feedback is questionable. And it is amazing how much clearer and better readable the verifiable geography is in the map if you do not show the boundaries.

I made an exception for the admin_level 2 boundaries because

• A fairly large percentage of admin_level 2 boundaries is physically manifested in some form – with either demarcations or even continuous structures like fences or walls. Although there is an annoying number of cases where admin_level 2 boundaries are not aligned to these physical manifestations it is fairly clear consensus that they should be. Hence mapper feedback is useful here.
• admin_level 2 boundaries have a much higher practical significance on the ground in most parts of the world than higher admin level boundaries.
• showing admin_level 2 boundaries has much less negative effect on map readability than the higher admin levels – both because they are less common and because they are shown as solid lines rather than dashed/dotted.

You can see in the AC-Style sample gallery how much this change improves overall map readability.