[Tagging] Rock outcrops in forest

Christoph Hormann osm at imagico.de
Mon May 28 12:50:41 UTC 2018

On Monday 28 May 2018, Ture PĂ„lsson wrote:
> >
> > landuse=residential and landuse:secondary=retail is what is
> > happening on the ground in some places.
> Random thoughts:
> Does this really buy anything compared to overlapping polygons
> (except, obviously, where the polygons are coincident, in which case
> it saves a polygon)?

It was not my intention to push this idea here or to advocate a 
particular use case.  If you as a mapper find it useful then use it, 
otherwise don't.

But creating two separate geometries to document different properties of 
the same real world objects, like the ground conditions in a 
wood/forest, is both pretty pointless and against OSM principles.  And 
semantically it is not clear in such a case which geometry represents 
the primary characterization of the area (what the area *is* so to 
speak) and which is a secondary characterization.

In general the meaning of overlapping polygon geometries is - with the 
exception of some situations where both geometries are clearly on 
different semantic levels - fairly undefined in OSM.  In many map 
styles including the standard style most land based area types that are 
rendered are sorted by size meaning that small polygons are interpreted 
as superseeding large geometries in meaning if they overlap.  But this 
is not a rule you can rely on.  So as a general rule for mapping: If 
you want to be clear and non-ambiguous you should typically avoid 
overlapping geometries of different feature types of the same or 
similar semantic levels.

Christoph Hormann

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