[Tagging] Using multipolygons to map bays in Alaska
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
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.
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