[Tagging] Using multipolygons to map bays in Alaska

Christoph Hormann osm at imagico.de
Thu Nov 15 16:47:45 UTC 2018


On Thursday 15 November 2018, Dave Swarthout wrote:
> Thanks for the feedback and the references to the previous
> discussions about this topic. The first reference to an earlier
> discussion on this list was particularly useful. From your email and
> that thread, I gather that you are opposed to mapping bays and
> straits as multipolygons. That comes through loud and clear.  But
> there are others who disagree. I think both sides make valid
> arguments and I tend to agree with those who say go ahead and use
> multipolygons. It seems to me a better choice but that choice is not
> without its drawbacks.

Yes, i completely recognize the incentive to map bays this way but i 
also think this is largely due to most not seeing the possibility to 
create high quality labeling based on nodes without polygons.  This is 
a dilemma that is ultimately only solvable by educating mappers to make 
informed decisions and by pressuring map designers and rendering 
framework developers to go the extra mile and not outsource their work 
to the mapper.

> Most 
> bays are fairly easy to outline with coastline and a simple way
> connecting the two extremes. Straits too are usually fairly easy to
> define spatially with the help of those maps. Straits are important
> geographic features and I think the use of multipolygons to define
> them is a useful methodology. Absolute accuracy is hard to attain but
> it doesn't matter all that much. What someone piloting a fishing boat
> in heavy weather wants to know is, am I in this bay or am I in the
> Shelikof Strait?  If the Shelikof Strait (240 km long and 40-48 km
> wide), is represented by a mere node, I doubt that it would be
> possible to know one way or the other.

For longer, branching and curved straits we have the established 
practice to map them with a linear way like we do for a river.  Same 
makes sense for fjords and fjord like bays.

For bays i would also like to use the example you gave:

https://www.openstreetmap.org/relation/8921978

At the western end the question obviously arises if Chickaloon Bay:

https://www.openstreetmap.org/node/369142797

is part of this or not.  It is mapped without but this is probably a 
subjective choice, possible for the purpose of desired label placement 
(no overlap means the labels can more easily be placed without 
collision).  This kind of situation is very common with bays because 
this kind of thing is usually just not verifiably decided when people 
start naming a bay a certain way.

In this case a node placed around here:

https://www.openstreetmap.org/node/2039514447

would reliably identify the bay together with the coastline but given 
the curved form at the eastern end a linear way could be clearer.

-- 
Christoph Hormann
http://www.imagico.de/



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