[Tagging] Using multipolygons to map bays in Alaska

Christoph Hormann osm at imagico.de
Thu Nov 15 11:08:53 UTC 2018

On Thursday 15 November 2018, Dave Swarthout wrote:
> [...]
> I was thinking it would be much easier and perhaps even better to
> just draw an approximate shape consisting of maybe 20 or 30 nodes,
> big enough to define the area and cause it to render, but easy to
> draw and without involving any multipolygons. The issue here is
> admittedly one I am pursuing to get these water bodies to render in a
> manner proportional to their size and I suspect that many will be
> against it on that basis alone. Still, I thought it worthwhile to
> mention my idea here and see what you think about it as a "shorthand"
> solution.

I think it is good you bring this up because many mappers have been 
doing exactly that without asking - See for example:


To put it right upfront:  This is a bad idea.  As you say the main 
motivation for doing this is to make a bay show up in the map.   
OSM-Carto has made the decision to incentivize this kind of mapping - 
and as i like to point out to derivate from its self declared goal to 
support mappers in consistent mapping towards steering mappers to map 
in a way that is convenient for style developers.

The 'polygons is universally the preferred way of mapping no matter if 
verifiable or not' and 'way_area equals cartographic importance' 
concepts have been meanwhile extended to natural=strait in OSM-Carto - 
thereby not only incentivizing against mapping with nodes but also 
against mapping with linear ways.

To be fair:  There are other map styles that do essentially the same so 
it is not appropriate to exclusively blame OSM-Carto for this but it is 
the only style that due to being rendered on OSMF infrastructure has a 
true obligation not to do this.

Mapping bays with polygons is always non-verifiable to a large extent.  
Mapping bays with polygons as you describe it above is always 
completely non-verifiable and amounts to pure (low quality) label 
painting which should not be done and should not be incentivized by 
maps with a mapper feedback goal.

If you want to generate high quality labeling for bays in maps what you 
need is the geometry of the waterbody the bay is part of (usually the 
coastline) and the location of the bay - which can easily be specified 
with a node in the way described on the wiki.  This allows for much 
higher quality labeling than a pretend-exact geometry either based on 
coastlines or not.  So the first thing you do with bay polygons for 
generating quality labeling is to derive a node location from the 
polygon and start from that - which makes the polygon drawing really 
kind of insane.

Long story short:  My suggestion is and has always been to map bays with 
nodes in those cases where this - together with the coastline - 
perfectly documents the verifiable information available on the 
geometry of the bay.  In other situations (which exist but are 
relatively rare) other verifiable modeling concepts can be considered. 
Drawing a coarse labeling polygon is not one of them.

Links to previous discussion on the matter:


Christoph Hormann

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