Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Kevin Kenny]

On Thu, Sep 13, 2018 at 10:02 AM Martin Koppenhoefer
 wrote:

> you’ll have to put the ridges to map the watersheds anyway, the catchment 
> basin is implicit with the waterways, coastlines and ridges.
>
> If there are names or other properties for the watersheds and catchment 
> basins in play, it could make sense to have dedicated objects nonetheless, I 
> agree.

> >
> > 2), while a ridge has to have a certain amount of slope to be called a 
> > ridge (perhaps at least 5 or 10% grade?), watershed boundaries are 
> > sometimes very shallow
>
>
> how can we observe those sheds in shallow areas? Can it be done on the ground 
> or does it require additional elevation data? Maybe in the context of shallow 
> land the sheds aren’t stable?

Definitely there are exorrheic wetlands and ponds in which the
watershed line is entirely indefinite. I know of a number of wet areas
that have distributaries into multiple major river basins; e.g., the
Grand Gorge area in the western Catskills drains into both the
Delaware and the Schoharie (and thence via the Mohawk to the Hudson);
the Preston Ponds area of the Adirondacks drains to both the Hudson
and to the Cold River (thence by the Raquette to the St Lawrence), and
so on.

The nearest the US has to an authoritative source is the National
Hydrography Database (NHD) which does have watershed boundaries, with
a hierarchical reference numbering system identifying them and tying
them to the 'reach codes' of the rivers that drain them. ('Reach code'
is a persistent identifier of up to 16 (?) digits, identifying
branches of a river from mouth to headwaters. (I've never investigated
what reach codes do with distributaries - I simply haven't needed to
know.)

There have been projects in the past to import NHD data in bulk.  I
was tempted to do it in my area once - I have the good fortune of
living in a place where the data quality of NHD is pretty good. While
some bulk imports from NHD have been highly successful, I stumbled on
conflation issues and abandoned the idea. I still occasionally
copy-n-paste a stream from NHD into JOSM, but that's always
onesie-twosies, and compared with data that are already in OSM and
with aerials. (Severe storms in the last decade or so have caused some
fairly major watercourses around here to shift, and NHD still hasn't
caught up.)

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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Martin Koppenhoefer]

sent from a phone

> On 13. Sep 2018, at 10:02, Joseph Eisenberg  
> wrote:
> 
> "do we really have to map this explicitly with relations? Can’t you already 
> see them from the waterway and ridge data?"
> 
> 1) Ridges are missing in many parts of the world, partially because they are 
> not rendered, but also because it might not be clear how they can be useful. 
> The presence of watershed relations, mapped along ridges, would encourage 
> other mappers to add the missing ridges. 


you’ll have to put the ridges to map the watersheds anyway, the catchment basin 
is implicit with the waterways, coastlines and ridges. 

If there are names or other properties for the watersheds and catchment basins 
in play, it could make sense to have dedicated objects nonetheless, I agree.



> 
> 2), while a ridge has to have a certain amount of slope to be called a ridge 
> (perhaps at least 5 or 10% grade?), watershed boundaries are sometimes very 
> shallow


how can we observe those sheds in shallow areas? Can it be done on the ground 
or does it require additional elevation data? Maybe in the context of shallow 
land the sheds aren’t stable?


WRT imports, if license is suitable and resolution satisfactory I would not 
generally oppose the idea of an import.


Cheers,
Martin 


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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Christoph Hormann]

On Thursday 13 September 2018, Joseph Eisenberg wrote:
> Christoph,
> So you believe the ridges are verifiable (and the network of
> waterways, I assume), but potentially parts of the watershed would
> not be verifiable because eg. terrain is too flat? 

There are many reasons why a the watershed structure can be practically 
non-verifiable on the ground.  Relatively flat topography is only one 
of them.  I would estimate that at least about 40 percent of the Earth 
land surfaces have a drainage structure that is practically 
non-verifiable (in the sense that independent estimates from several 
mappers would not converge to the same geometry).  The percentage would 
be even higher if you want it to be verifiable through remote mapping.

> I was thinking 
> that in fairly flat areas it is still possbile to see which way water
> flows in drainage channels, and it's often possible to find the
> highest line throught he terrain when surveying.

Nothing speaks against mapping physically observable features like 
drainage channels but mapping additional abstract geometries derived 
from them in OSM makes little sense IMO.

Note in flat areas with artificial drainage channels the actual drainage 
structure can be extremely complex.

> Would these examples be verifiable?
>
> [...]

Not having first hand knowledge of these cases means i can't really 
tell.  A waterway is a geometry directly physically observable on the 
ground.  The watershed divide is an abstract geometry OTOH. You see a 
ridge and *assume* that water from one side of the ridge flows into a 
certain drainage system and on the other side it flows into a different 
one.  But you don't observe this.  You might have a simple case where 
this seems obvious but the fundamental problem is still there.  In 
humid climate areas you can make the mapping more reliable by first 
diligently mapping the waterway network in the whole area but then what 
is the point in mapping the watershed divides in addition?

Also note in priciple watersheds form an infinitely deep hierarchy of 
geometries.  To be able to practically map these you would have to 
define a discretization system in this hierarchy which would be an 
arbitrary up-front decision with no basis in the physical world.

> I value your opinion Christoph, because I hoped this relation might
> encourage more complete mapping of ridges, watersheds and drainage
> basins, thus making it easier to render good maps, eg
> http://www.imagico.de/map/water_generalize_en.php

If you want to facilitate better maps w.r.t. hydrography the best way 
would be to put all the time and energy you might put into mapping 
watersheds into mapping the waterway network. Priorities should be:

* correct connectivity
* correct distinction onto artificial and natural
* correct flow direction
* completeness and unifomity in detail of mapping

Where these conditions are fulfilled processing the waterbody data for 
accurate maps is orders of magnitude easier than elsewhere.  Compared 
to that the would-be gain of having watershed geometries available in 
addition would be relatively small.

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

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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Joseph Eisenberg]

"In fact this does occur, a river can disappear into the sand!
And some lakes have no outflow."
Right, these are called an "endorheic basin"; an area where the water flow
does not reach the ocean: https://en.wikipedia.org/wiki/Endorheic_basin
In one of my examples above, the river disappears into limestone sinkholes
but then reappears later. This is one of the reasons that it can be
difficult to automatically calculate stream networks and watersheds, if
there are no waterway relations. The presence of endorheic basins and
vanishing streams in deserts and limestone areas is one of the reasons I
believe a watershed or drainage basin relation would be useful.

On Thu, Sep 13, 2018 at 7:55 PM Warin <61sundow...@gmail.com> wrote:

> I believe some waterways in Australia will flow away from wherever the
> rain happens to fall ...
> That is a produce not just of the flatness of the terrain but also to the
> quantity of rain - there can be 5 years of rainfall delivered in a single
> day.
>
> Someone has put in the Australian Great Dividing Range... fortunately it
> does not render as it is very rough data. And there is no motivation to fix
> it .. as it does not render most are unaware of it within OSM.
>
> On 13/09/18 20:12, Joseph Eisenberg wrote:
>
> Christoph,
> So you believe the ridges are verifiable (and the network of waterways, I
> assume), but potentially parts of the watershed would not be verifiable
> because eg. terrain is too flat? I was thinking that in fairly flat areas
> it is still possbile to see which way water flows in drainage channels, and
> it's often possible to find the highest line throught he terrain when
> surveying. Also, open topographical map sources and open source elevation
> data (eg SRTM) would be the main way to determine this. Would it be ok to
> map watersheds where they are verifiable?
>
> Would these examples be verifiable?
>
> Wolo river is 99% surrounded by steep ridges; good example?:
> https://www.openstreetmap.org/relation/8687500
> 
>  see https://www.opentopomap.org/#map=13/-3.7955/138.9242
>
> The Ibele river may be questionable in the flat valley
> https://www.openstreetmap.org/relation/8687462
> 
>  see https://www.opentopomap.org/#map=12/-4.0620/138.7477
>
> Uwe river is mainly surrounded by steep ridges. For the part in town
> verification depends on seeing which way water flows in open drainage
> ditches along streets; https://www.openstreetmap.org/relation/8687464
> 
>  see https://www.opentopomap.org/#map=11/-4.2105/138.7910
>
> Tagi river is 95 surrounded by ridges:
> https://www.openstreetmap.org/relation/8687463 ;
>
> Lake Habema is 98% surrounded by ridges:
> https://www.openstreetmap.org/relation/8688506 ;
> https://www.opentopomap.org/#map=13/-4.1419/138.6722
>
> I would be happy to include a warning in the proposal and wiki that not
> every watershed can be mapped in OSM. Only those in terrain where the
> dividing line is obvious and the direction of water drainage is clear. So
> no watersheds should be mapped in wetlands, flat farmland, developed urban
> areas, etc.
>
> If the issue is the lines through flatter terrain, we could have the
> watershed be a non-closed line which only connected ridges. (Personally I
> feel there are places where the watershed is obvious, yet don't qualify as
> a "ridge", which could also be included as part of the relation). If there
> was no requirement to make a closed way from the segments, this would
> remove the temptation to draw non-verifiable lines across flat land.
>
> I value your opinion Christoph, because I hoped this relation might
> encourage more complete mapping of ridges, watersheds and drainage basins,
> thus making it easier to render good maps, eg
> http://www.imagico.de/map/water_generalize_en.php
>
>
> I note the expatiation the a river will not stop in the middle of nowhere.
> In fact this does occur, a river can disappear into the sand!
> And some lakes have no outflow.
>
>
> On Thu, Sep 13, 2018 at 6:14 PM Christoph Hormann  wrote:
>
>> On Thursday 13 September 2018, Joseph Eisenberg wrote:
>> > Relations of type=watershed are currently used over 2000 times and
>> > there is a descriptive Wiki page but no proposal. (
>> > https://wiki.openstreetmap.org/wiki/Relation:watershed)
>> >
>> > It would be useful to have a relation that showed drainage divides
>> > (aka watersheds) and drainage basins (the network of streams and
>> > rivers draining into a water body or waterway)
>>
>> Watershed divides are an abstract non-physical concept that is generally
>> not verifiable in practical mapping - there are cases where they are
>> (because they evidently coincide with physical features like ridges)
>> but there are huge parts of the world where they are not and you would
>> only try to 

Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Warin]

I believe some waterways in Australia will flow away from wherever the 
rain happens to fall ...
That is a produce not just of the flatness of the terrain but also to 
the quantity of rain - there can be 5 years of rainfall delivered in a 
single day.


Someone has put in the Australian Great Dividing Range... fortunately it 
does not render as it is very rough data. And there is no motivation to 
fix it .. as it does not render most are unaware of it within OSM.


On 13/09/18 20:12, Joseph Eisenberg wrote:

Christoph,
So you believe the ridges are verifiable (and the network of 
waterways, I assume), but potentially parts of the watershed would not 
be verifiable because eg. terrain is too flat? I was thinking that in 
fairly flat areas it is still possbile to see which way water flows in 
drainage channels, and it's often possible to find the highest line 
throught he terrain when surveying. Also, open topographical map 
sources and open source elevation data (eg SRTM) would be the main way 
to determine this. Would it be ok to map watersheds where they are 
verifiable?


Would these examples be verifiable?

Wolo river is 99% surrounded by steep ridges; good example?: 
https://www.openstreetmap.org/relation/8687500 
 see 
https://www.opentopomap.org/#map=13/-3.7955/138.9242


The Ibele river may be questionable in the flat valley 
https://www.openstreetmap.org/relation/8687462 
 see 
https://www.opentopomap.org/#map=12/-4.0620/138.7477


Uwe river is mainly surrounded by steep ridges. For the part in town 
verification depends on seeing which way water flows in open drainage 
ditches along streets; https://www.openstreetmap.org/relation/8687464 
 see 
https://www.opentopomap.org/#map=11/-4.2105/138.7910


Tagi river is 95 surrounded by ridges: 
https://www.openstreetmap.org/relation/8687463 ;


Lake Habema is 98% surrounded by ridges: 
https://www.openstreetmap.org/relation/8688506 ; 
https://www.opentopomap.org/#map=13/-4.1419/138.6722


I would be happy to include a warning in the proposal and wiki that 
not every watershed can be mapped in OSM. Only those in terrain where 
the dividing line is obvious and the direction of water drainage is 
clear. So no watersheds should be mapped in wetlands, flat farmland, 
developed urban areas, etc.


If the issue is the lines through flatter terrain, we could have the 
watershed be a non-closed line which only connected ridges. 
(Personally I feel there are places where the watershed is obvious, 
yet don't qualify as a "ridge", which could also be included as part 
of the relation). If there was no requirement to make a closed way 
from the segments, this would remove the temptation to draw 
non-verifiable lines across flat land.


I value your opinion Christoph, because I hoped this relation might 
encourage more complete mapping of ridges, watersheds and drainage 
basins, thus making it easier to render good maps, eg 
http://www.imagico.de/map/water_generalize_en.php


I note the expatiation the a river will not stop in the middle of nowhere.
In fact this does occur, a river can disappear into the sand!
And some lakes have no outflow.



On Thu, Sep 13, 2018 at 6:14 PM Christoph Hormann > wrote:


On Thursday 13 September 2018, Joseph Eisenberg wrote:
> Relations of type=watershed are currently used over 2000 times and
> there is a descriptive Wiki page but no proposal. (
> https://wiki.openstreetmap.org/wiki/Relation:watershed)
>
> It would be useful to have a relation that showed drainage divides
> (aka watersheds) and drainage basins (the network of streams and
> rivers draining into a water body or waterway)

Watershed divides are an abstract non-physical concept that is
generally
not verifiable in practical mapping - there are cases where they are
(because they evidently coincide with physical features like ridges)
but there are huge parts of the world where they are not and you
would
only try to estimate them based on already existing data.

In short: This is not something you can reasonably map in OSM.

-- 
Christoph Hormann

http://www.imagico.de/



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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Joseph Eisenberg]

(Sorry, Lake Habema is https://www.openstreetmap.org/relation/8688509)

On Thu, Sep 13, 2018 at 7:12 PM Joseph Eisenberg 
wrote:

> Christoph,
> So you believe the ridges are verifiable (and the network of waterways, I
> assume), but potentially parts of the watershed would not be verifiable
> because eg. terrain is too flat? I was thinking that in fairly flat areas
> it is still possbile to see which way water flows in drainage channels, and
> it's often possible to find the highest line throught he terrain when
> surveying. Also, open topographical map sources and open source elevation
> data (eg SRTM) would be the main way to determine this. Would it be ok to
> map watersheds where they are verifiable?
>
> Would these examples be verifiable?
>
> Wolo river is 99% surrounded by steep ridges; good example?:
> https://www.openstreetmap.org/relation/8687500
> 
>  see https://www.opentopomap.org/#map=13/-3.7955/138.9242
>
> The Ibele river may be questionable in the flat valley
> https://www.openstreetmap.org/relation/8687462
> 
>  see https://www.opentopomap.org/#map=12/-4.0620/138.7477
>
> Uwe river is mainly surrounded by steep ridges. For the part in town
> verification depends on seeing which way water flows in open drainage
> ditches along streets; https://www.openstreetmap.org/relation/8687464
> 
>  see https://www.opentopomap.org/#map=11/-4.2105/138.7910
>
> Tagi river is 95 surrounded by ridges:
> https://www.openstreetmap.org/relation/8687463 ;
>
> Lake Habema is 98% surrounded by ridges:
> https://www.openstreetmap.org/relation/8688506 ;
> https://www.opentopomap.org/#map=13/-4.1419/138.6722
>
> I would be happy to include a warning in the proposal and wiki that not
> every watershed can be mapped in OSM. Only those in terrain where the
> dividing line is obvious and the direction of water drainage is clear. So
> no watersheds should be mapped in wetlands, flat farmland, developed urban
> areas, etc.
>
> If the issue is the lines through flatter terrain, we could have the
> watershed be a non-closed line which only connected ridges. (Personally I
> feel there are places where the watershed is obvious, yet don't qualify as
> a "ridge", which could also be included as part of the relation). If there
> was no requirement to make a closed way from the segments, this would
> remove the temptation to draw non-verifiable lines across flat land.
>
> I value your opinion Christoph, because I hoped this relation might
> encourage more complete mapping of ridges, watersheds and drainage basins,
> thus making it easier to render good maps, eg
> http://www.imagico.de/map/water_generalize_en.php
>
> On Thu, Sep 13, 2018 at 6:14 PM Christoph Hormann  wrote:
>
>> On Thursday 13 September 2018, Joseph Eisenberg wrote:
>> > Relations of type=watershed are currently used over 2000 times and
>> > there is a descriptive Wiki page but no proposal. (
>> > https://wiki.openstreetmap.org/wiki/Relation:watershed)
>> >
>> > It would be useful to have a relation that showed drainage divides
>> > (aka watersheds) and drainage basins (the network of streams and
>> > rivers draining into a water body or waterway)
>>
>> Watershed divides are an abstract non-physical concept that is generally
>> not verifiable in practical mapping - there are cases where they are
>> (because they evidently coincide with physical features like ridges)
>> but there are huge parts of the world where they are not and you would
>> only try to estimate them based on already existing data.
>>
>> In short: This is not something you can reasonably map in OSM.
>>
>> --
>> Christoph Hormann
>> http://www.imagico.de/
>>
>> ___
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>>
>
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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Joseph Eisenberg]

Christoph,
So you believe the ridges are verifiable (and the network of waterways, I
assume), but potentially parts of the watershed would not be verifiable
because eg. terrain is too flat? I was thinking that in fairly flat areas
it is still possbile to see which way water flows in drainage channels, and
it's often possible to find the highest line throught he terrain when
surveying. Also, open topographical map sources and open source elevation
data (eg SRTM) would be the main way to determine this. Would it be ok to
map watersheds where they are verifiable?

Would these examples be verifiable?

Wolo river is 99% surrounded by steep ridges; good example?:
https://www.openstreetmap.org/relation/8687500

 see https://www.opentopomap.org/#map=13/-3.7955/138.9242

The Ibele river may be questionable in the flat valley
https://www.openstreetmap.org/relation/8687462

 see https://www.opentopomap.org/#map=12/-4.0620/138.7477

Uwe river is mainly surrounded by steep ridges. For the part in town
verification depends on seeing which way water flows in open drainage
ditches along streets; https://www.openstreetmap.org/relation/8687464

 see https://www.opentopomap.org/#map=11/-4.2105/138.7910

Tagi river is 95 surrounded by ridges:
https://www.openstreetmap.org/relation/8687463 ;

Lake Habema is 98% surrounded by ridges:
https://www.openstreetmap.org/relation/8688506 ;
https://www.opentopomap.org/#map=13/-4.1419/138.6722

I would be happy to include a warning in the proposal and wiki that not
every watershed can be mapped in OSM. Only those in terrain where the
dividing line is obvious and the direction of water drainage is clear. So
no watersheds should be mapped in wetlands, flat farmland, developed urban
areas, etc.

If the issue is the lines through flatter terrain, we could have the
watershed be a non-closed line which only connected ridges. (Personally I
feel there are places where the watershed is obvious, yet don't qualify as
a "ridge", which could also be included as part of the relation). If there
was no requirement to make a closed way from the segments, this would
remove the temptation to draw non-verifiable lines across flat land.

I value your opinion Christoph, because I hoped this relation might
encourage more complete mapping of ridges, watersheds and drainage basins,
thus making it easier to render good maps, eg
http://www.imagico.de/map/water_generalize_en.php

On Thu, Sep 13, 2018 at 6:14 PM Christoph Hormann  wrote:

> On Thursday 13 September 2018, Joseph Eisenberg wrote:
> > Relations of type=watershed are currently used over 2000 times and
> > there is a descriptive Wiki page but no proposal. (
> > https://wiki.openstreetmap.org/wiki/Relation:watershed)
> >
> > It would be useful to have a relation that showed drainage divides
> > (aka watersheds) and drainage basins (the network of streams and
> > rivers draining into a water body or waterway)
>
> Watershed divides are an abstract non-physical concept that is generally
> not verifiable in practical mapping - there are cases where they are
> (because they evidently coincide with physical features like ridges)
> but there are huge parts of the world where they are not and you would
> only try to estimate them based on already existing data.
>
> In short: This is not something you can reasonably map in OSM.
>
> --
> Christoph Hormann
> http://www.imagico.de/
>
> ___
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>
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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Christoph Hormann]

On Thursday 13 September 2018, Joseph Eisenberg wrote:
> Relations of type=watershed are currently used over 2000 times and
> there is a descriptive Wiki page but no proposal. (
> https://wiki.openstreetmap.org/wiki/Relation:watershed)
>
> It would be useful to have a relation that showed drainage divides
> (aka watersheds) and drainage basins (the network of streams and
> rivers draining into a water body or waterway)

Watershed divides are an abstract non-physical concept that is generally 
not verifiable in practical mapping - there are cases where they are 
(because they evidently coincide with physical features like ridges) 
but there are huge parts of the world where they are not and you would 
only try to estimate them based on already existing data.

In short: This is not something you can reasonably map in OSM.

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

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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Joseph Eisenberg]

"do we really have to map this explicitly with relations? Can’t you already
see them from the waterway and ridge data?"

1) Ridges are missing in many parts of the world, partially because they
are not rendered, but also because it might not be clear how they can be
useful. The presence of watershed relations, mapped along ridges, would
encourage other mappers to add the missing ridges.

2), while a ridge has to have a certain amount of slope to be called a
ridge (perhaps at least 5 or 10% grade?), watershed boundaries are
sometimes very shallow. If you look at the examples I posted, you can see
that the watershed boundaries in the valley are not obvious without looking
at topographical data or surveying in person (I used opentopomap and
survey). Some of the high watershed on plateaus also have the same problem.
But the watershed or basin divide line is verifiable by finding the highest
between basins.

3) Watersheds may already be available as open source data. Ridges must be
drawn with increasing elevation and cannot meet waterways, so it would be
wrongt to import watershed boundary data directly as a serious of ridges.
But with this relation it would be possible to import a watershed as a way
or ways with the role "outer". Hopefully mappers would later add
natural=ridge tags, after breaking up the ways at saddles and peaks so that
they can be drawn in the correct direction (uphill).

4) In my area, aerial imagery over mountain is poor due to cloud cover, but
the SRTM elevation data is pretty good. So I can easily draw the watershed
boundary line by following topography, but it isn't possible to add every
segement of river and stream in the mountains. I believe this would make it
difficult to calculate the correct watershed boundaries

Many segments of waterways are incomplete (especially outside of Europe).
Sometimes this is incomplete data, but it can also be a real feature. For
example, the Wolo River (https://www.openstreetmap.org/relation/8687500)
goes underground in a limestone area and reappears kilometers laters. There
is even a sharp ridge between the two valleys. There are at least 2 or 3
other big rivers in my area that do the same thing. Analyzing the ridges
and waterways alone would miss this.

I started thinking about this after seeing how Imagico used ridges to help
separate waterways and find bad waterway data, and he mentioned that there
are not enough ridges, and they are also sometimes unreliable (
http://blog.imagico.de/river-generalization/). The existence of a watershed
relation would show that someone took the time to check all the ridges.
Also, the most prominent ridgeline is not always the same as the watershed
boundary. When a mapper is making a watershed as a complete boundary, they
will have to complete the gaps, and will be encouraged to find out the
basin divide line even in flattish areas.

Joseph Eisenberg

PS: Any thought about the name of the key and value, and the type of
relation that is best; boundary or not?


On Thu, Sep 13, 2018 at 4:06 PM Martin Koppenhoefer 
wrote:

> do we really have to map this explicitly with relations? Can’t you already
> see them from the waterway and ridge data? IIRR, people have been rendering
> maps for these in the past by just analyzing existing waterway data, no
> need for explicit watersheds.
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Re: [Tagging] Watershed or Drainage Basin relation draft proposal

[Martin Koppenhoefer]

do we really have to map this explicitly with relations? Can’t you already see 
them from the waterway and ridge data? IIRR, people have been rendering maps 
for these in the past by just analyzing existing waterway data, no need for 
explicit watersheds.
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