Mapping fjords with arms

I wanted to map few fjords in Norway, and I’m not sure if I should consider the arms of a fjord, a part of this fjord. For example I mapped Storfjorden (Relation: ‪Storfjorden‬ (‪16124483‬) | OpenStreetMap) and Norangsfjorden (Relation: ‪Norangsfjorden‬ (‪16124484‬) | OpenStreetMap) which are arms of Hjørundfjorden. Hjørundfjorden I mapped like this: Relation: ‪Hjørundfjorden‬ (‪16124482‬) | OpenStreetMap - I included Storfjorden and Norangsfjorden in the relation of Hjørundfjorden.
The question is - is it the correct way to map fjords? Or should I exclude Norangsfjorden and Storfjorden from the Hjørundfjorden relation? When I looked how other fjords are mapped, I have seen both ways are used.


I would map the three fjords separately/non-overlapping.

Map the fjord with a simple way (line) along the centre of the fjord, or a node in the middle of the fjord if it is small, or a simplified polygon with a few nodes within the water area. See the wiki for details.

Please do not create relations consisting of coastline ways. These relations become large and complex (including islands), breaks easily and makes it extremely difficult to maintain the coastline.

I checked the wiki before creating these relations and it explicitly allows to map fjords as a multipolygon using coastline.

I prefer overlapping multipolygons in cases where one fjord is recognized as an arm of a larger fjord. I’d just like you to be careful in verifying that there is such a parent/child relationship. There are examples where this might break, for instance the fjords around Relation: ‪Osterøy‬ (‪4494611‬) | OpenStreetMap. Multipolygons - particularly big ones - are harder to maintain, for sure, as @NKA points out, but done the right way I feel they provide more “accurate” map data than a line. IMO, I think it is a worthwhile task for us to try to figure out how to easily maintain the coastline in the presence of multipolygon relations.

How can I verify such a parent/child relationship?

Read up on relevant sources. If Fjord A is described as an arm of Fjord B, for instance on (preferably Norwegian language) Wikipedia or, or in other reputable sources, then it’s probably fine to map them as overlapping multipolygons.

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