[Tagging] Using multipolygons to map bays in Alaska

Christoph Hormann osm at imagico.de
Sat Nov 17 10:54:39 UTC 2018

On Saturday 17 November 2018, Kevin Kenny wrote:
> [...]
> If I cannot be given an answer to such a concrete problem, I will not
> accept that the reason is that I am too stupid or unskilled to
> understand your superior wisdom. [...]

To be clear i am not attempting to somehow proove something in this 
discussion.  I provide advise how to approach the verifiable mapping of 
bays.  That advise is and has always been (quoting here from my first 

"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"

The existence of situations where this is not the case would not in any 
way invalidate my advise.  And mappers following my advise or not for 
whatever reason is perfectly fine for me.

That being said looking at the Jamaica Bay situation - i think we can 
agree that this is a hard case for labeling independent of how the bay 
is mapped in particular also on maps showing the entire bay because it 
is next to impossible to label the bay with a single label without 
undesirable collisions with other features.

In contrast to other bay labeling cases i have not actually dealt with 
this kind of situation before.  Still i think this is solvable and if 
someone would ask to contract me for developing a solution for this i 
would be highly interested because this would likely also touch several 
problems that are of significant use in cartographic generalization in 
general.  A significant part of the problem is that with the current 
node placement it is borderline ambiguous if Jamaica Bay refers to the 
whole bay and Grassy Bay is only a part of it or if both refer to parts 
of the bay.  To relatively robustly identify that is likely the tricky 

Apart from that the key here would be to identify the various islands 
within the bay to be within the bay and not part of the limitation of 
the bay.  Once this is done and you have also analyzed the various 
sub-bays in the east (which look all relatively simple) you know the 
water at the western edge of your map sheet belongs to Jamaica Bay and 
the problem is solved.

Not sure if that convinces you - as said my argument is not based on the 
claim that nodes are universally sufficient to document verifiable 
knowledge about bays.

But in any case i want to be clear about one thing:

> If it is a project that aspires to describe the whole world, it will 
surely have failed in that place.

This is not what OpenStreetMap aims to do.  This might be wikipedia's 
goal but OSM tries to collect verifiable knowledge about the geography 
of the world.  That the existence of Jamaica Bay is part of that is not 
in dispute but still it is important to make that distinction.

Christoph Hormann

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