[Tagging] Water in bays, harbors, etc.

[Christoph Hormann]

On Tuesday 06 September 2016, Kevin Kenny wrote:
> I'll be following this discussion with interest, because I suspect
> that a good many mappers have the same approach that I do in
> estuarine areas: "don't mess with the coastline, you're too likely to
> break something."

Note this is acutally not something you have to fear much, at least when 
you edit with JOSM.  When the coastline is not updated this is usually 
either:

- a beginner editing who has never heard about the special problems of 
coastline mapping in OSM.
- deliberate, technically correct edits that are large enough to trigger 
the error detection heuristics. 
- unqualified imports.

In other words: if you are aware of the problem you are unlikely to 
create a formal error while doing normal editing of the coastline even 
if you don't shy away from coastline edits.

> Moreover, the 
> coastline often seems to follow the barrier islands, with the back
> bays, estuaries, and other waterways not accounted as part of the
> ocean, even if the water is salt. There will always be confusion in
> that area. It would surely be wrong to label the Hudson River as
> merely an arm of the Atlantic Ocean, even if the water is brackish
> for fifty kilometers or more upriver, and the tide is measurable for
> another hundred km beyond that, so there has to be some room for
> judgment.

For river mouths there is a proposal suggesting rough limits:

http://wiki.openstreetmap.org/wiki/Proposed_Features/Coastline-River_transit_placement

This more or less represents the practical mapping consensus - there are 
only a handful of larger rivers worldwide that do not comply with the 
limits suggested there.

There are a number of places where people have started mapping 
significant parts of coastal waters as polygons like here:

http://www.openstreetmap.org/relation/5405670
http://www.openstreetmap.org/relation/3801633

but this is not widely accepted and will cause problems for data users.

By the way you do not usually have to be that nit-picky about the exact 
definition of the water level.  Targeting the average high water line 
during the normal daily tidal cycle is usually close enough and as you 
already mentioned much better than current practice in many cases.  
Note spring tide level specifically does not mean storm water levels.  
So areas subject to storm flooding are not outside the coastline.

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