[Tagging] Using multipolygons to map bays in Alaska

Christoph Hormann osm at imagico.de
Thu Nov 15 16:49:58 UTC 2018

On Thursday 15 November 2018, Frederik Ramm wrote:
> Agree on the node idea, but it would have to include some size
> signifier (and I think someone recently tried to add a "sqm" tag to
> water body nodes for that purpose which I also criticised...). I
> don't think you are recommending a relation that includes the actual
> coastline and the label node, but if you do then I am against that
> because I don't want every coastline to be part of 10 relations in
> the end.

No, mapping explicitly with a relation a spatial relationship that is 
already implicitly mapped with the geometries is nonsense.

I should probably add a bit of technical background - which i mentioned 
quickly in one of the github issues i linked to but which might not be 
universally clear.

Most map styles like OSM-Carto do not have the coastline data in the 
rendering database.  They render the coastline from a separate 
shapefile.  Therefore the coastline is not readily available to the map 
designer for making rendering decisions.  But that is a specific 
limitation of the map rendering setup widely used.  I hope everyone 
agrees it is a bad idea to nudge mappers all over the world to map 
things in a certain way because of a specific limitation like this in 
certain map rendering toolchains.

If you have the coastline available with the other data in a rendering 
database you can take various approaches to assess the size of the bay 
from the node location and the coastline geometry.  If you want to do 
this on the fly you need to be careful about query performance.  
Alternatives are to pre-calculate such information or to preprocess the 
coastline data to allow faster assessment.  This is all no rocket 
science although i admit that for the typical map designer or geodata 
engineer who is used to think polygon centered this requires some 
amount of outside-the-box thinking.

Christoph Hormann

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