Re: [Tagging] Water in bays, harbors, etc.

[Kevin Kenny]

On Tue, Sep 6, 2016 at 3:53 PM, Christoph Hormann 
wrote:

> 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.
>

I've actually learnt that, and even repaired the coastline a time or two. I
exaggerated when I said, 'stay away from the coastline,' it's more 'if you
get started editing the coastline, expect to spend an unpredictable amount
of time repairing the topology before JOSM will shut up about it..' (Which
is most likely a good thing, but it's annoying when it happens. Usually, it
means that I'm tidying someone else's mess.)


> 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.
>

The Hudson River is one of them, but the locals would be somewhat
astonished if the "seacoast" extended as far as
http://www.openstreetmap.org/#map=14/41.3070/-73.9655 (the typical location
of the salt front in conditions of normal flow, and a typical place above
which the river current begins to predominate), to say nothing of
http://www.openstreetmap.org/#map=16/42.7518/-73.6875 (the dam that is the
tidal limit). For what it's worth, NHD classifies that entire reach as
'estuary', which has caused rendering problems for CalTopo and TopOSM,
among others. The locals know it as the "Hudson River" all the way down to
the southern tip of Manhattan and do not think of it as an arm of the sea,
geology be damned.


> 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.
>

Indeed. Where I've edited, I've tried to follow local practice and make the
foreshore the multipolygon, rather than having water polygons.
http://www.openstreetmap.org/relation/4611285 is an example. (Note that
others created that particular relation; I modified it only because I
needed to reuse shoreline as the boundary of a state park. It got rather
nasty because someone ahead of me had done a forced repair to make the
coastline continuous and had broken the foreshore multipolygon in the
process. Drive-by mapping at its finest - but forgivable, because a broken
coastline is a continent-level emergency while unclosed ways in foreshore
polygons are local problems.)


> 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.
>

Right. I'm talking about neighbourhoods like
https://si.wsj.net/public/resources/images/NY-CQ681_MNYBLO_P_20131024232207.jpg
where the houses are built well above the ground on piers, and the streets
may well be awash at the new and full Moon even in good weather,
particularly if the wind is offshore. That house with the turret has a
stairway of ten or twelve steps going up to its front porch. You can see
the pilings under the house with the flagpole. (When I was growing up, it
hardly occurred to me that people in other places never needed to put on
high rubber boots to cross the street, or that raking flotsam out of the
front yard wasn't a routine chore for most!)
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Re: [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/

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Re: [Tagging] Water in bays, harbors, etc.

[Kevin Kenny]

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."

My understanding is that the coastline is *supposed* to follow the
high-water line of the mean spring tide of a 19-year Metonic cycle. In the
field, though, what I see mapped is something closer to mean daily high
water, or even lower, because otherwise most of the renderers would place
the entire foreshore under water. 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.

If the natural=coastline followed the mean high water of the Metonic cycle,
as documented in http://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline,
many of the residential areas in a waterfront community like my native area
of http://www.openstreetmap.org/#map=15/40.6160/-73.7555 [1] would render
as being underwater - they routinely have water in the streets for an hour
or two at the spring tide, particularly if the wind is offshore.
Technically, they are indeed 'outside' the coastline, but practically, that
description does not make sense. All the buildings are on piers, typically
a couple of metres above grade, and people don't think of themselves as
living under water. They just keep an almanac close at hand to know when
the road will be flooded.

-- 
73 de ke9tv/2, Kevin

[1] People often ask, "why would people build in an area that's so very
subject to flooding?" The answer is simple: "because it's hard to put a
seaport anywhere but on the ocean."  And now I see that I have another
thing on my ever-growing list of projects. I see a couple of streets on the
map there that have been abandoned since the hurricane of 1960 and have
been underwater since the 1970s at the very least. They are no doubt 'TIGER
turds.'

On Sun, Aug 21, 2016 at 11:48 AM, Tod Fitch  wrote:

> During a day kayaking I noticed that the OSM map in the area was less than
> perfect so I thought I’d see what I could do to improve it. But I have not
> worked on this type of feature very much and am a little uncertain of some
> tagging details.
>
> The area is a “back bay”, a tidal area with some channels and tidal
> wetlands a bit inland from a developed small boat harbor. In this area the
> land/water boundary has been tagged with “natural=coastline”[1]. When
> looking at other similar areas I’ve been to, it seems that the
> “natural=coastline” is not taken as far inland and the harbor and estuary
> areas are tagged with "natural=water”[2]. I am wondering which is the
> preferred method of tagging.
>
>
> Thanks!
>
> [1] http://www.openstreetmap.org/#map=16/33.6201/-117.8968
> [2] http://www.openstreetmap.org/#map=16/36.8100/-121.7859
>
>
>
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[Tagging] Water in bays, harbors, etc.

[Tod Fitch]

During a day kayaking I noticed that the OSM map in the area was less than 
perfect so I thought I’d see what I could do to improve it. But I have not 
worked on this type of feature very much and am a little uncertain of some 
tagging details.

The area is a “back bay”, a tidal area with some channels and tidal wetlands a 
bit inland from a developed small boat harbor. In this area the land/water 
boundary has been tagged with “natural=coastline”[1]. When looking at other 
similar areas I’ve been to, it seems that the “natural=coastline” is not taken 
as far inland and the harbor and estuary areas are tagged with 
"natural=water”[2]. I am wondering which is the preferred method of tagging.


Thanks!

[1] http://www.openstreetmap.org/#map=16/33.6201/-117.8968
[2] http://www.openstreetmap.org/#map=16/36.8100/-121.7859



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