[OSM-dev] Generalisation

[Christoph Hormann]

On Monday 16 April 2018, Martin Koppenhoefer wrote:
> AFAIK, in the osm-carto style there is no generalization on live data
> for performance reasons (because of continuous data updates via
> minutely diffs). There are some precomputed / extracted data files
> though, some of which contain generalized (simplified) data. These
> are all "external" sources:

I think this requires some clarification.

There are different definitions of what kind of operations and processes 
you call "generalization".  But typically selection is considered a 
generalization operation meaning that most of what is shown at all but 
the highest zoom levels is shown with some level of generalization.  
Most selection is rather primitive - based on fixed zoom level 
thresholds, the dreaded way_area filtering etc.  There are very limited 
attempts at some additional sophistication, like for populated place 
rendering at low to mid zoom levels.

When talking about explicit geometric generalization while specifically 
exclusing lossy vector data compression (which is widely 
mischaracterized as generalization) the Natural Earth boundaries at 
z=1-3 is currently the only case.

Lossy vector data compression in addition is currently/historically used 
in the following places:

* Coastlines at z8-9 - the visual impact of this was rather small but 
visible in some areas (in particular with small islands) if you kept an 
eye on it at the transit from z9 to z10.  This has been the case for a 
long time but is essentially superseded by:
* Coastlines at z0-9 since
https://github.com/gravitystorm/openstreetmap-carto/pull/3065
* The administrative boundaries at z4+ since
https://github.com/gravitystorm/openstreetmap-carto/pull/3103

You need to keep in mind that both geometric generalization and lossy 
vector data compression operations are largely incompatible with the 
current goals of OSM-Carto:

https://github.com/gravitystorm/openstreetmap-carto/blob/master/CARTOGRAPHY.md

-- 
Christoph Hormann
http://www.imagico.de/