Map a divide?

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Map a divide?

Kevin Kenny-3
In some maps that I render, I want to show the divide between a couple
of major river basins. (I have a good DEM for the area in question and
can derive the line readily.)

In light of the recent thread on topographic prominence, I wonder if
this is sufficiently interesting information at least to push it to
OSM. (If not, that's fine, I have a PostGIS database and a bucket of
shapefiles and know what to do.)

If it is sufficiently interesting, the question then arises: how to map/tag it?

'natural=ridge' comes to mind, and the divide in question has a local
name. (The 'Catskill Divide' separates the basins of the Hudson and
Delaware Rivers.) This approach appears to run into problems, as I
read the Wiki. I see:

> The way should connect saddle points and peaks, and the arrows should point upwards.

That may be all right for a ridge ascending the flank of a single
mountain, but what I'm talking about is the spine of a range, with the
ridge traversing dozens of named peaks. Even with a single mountain,
if there are false summits, the arrows on a single way cannot point
upward all the time! (And the wiki is clear that the

Do I misread, and should the reading instead simply be that the
arrowhead should be higher than the arrow tail? In that case, I could
break the divide into two ways, with a common endpoint at the highest
summit in the range.

Consider this a low-priority item. I have (or will have - there is a
bit of debugging yet) the data. I know how to render them. I'm happy
enough with a shapefile or a private PostGIS table if others aren't
interested.

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Re: Map a divide?

Tobias Zwick
The definition in the wiki is a bit contradictory, in my opinion. On the one side, it states the thing about that the arrows should point away from the saddle point towards the peaks, like steps or a oneway street, on the other side it describes a ridge to connect several peaks and saddle points. Tagging the whole ridge as one way would be impossible if you followed that "arrow points upward" rule.

There is also no mention of that rule in the original approved proposal. Looking at the history of the article, that rule was added in January 2018, following a short, well "discussion" about how a ridge could be rendered.
The change made amounts to a incompatible, as your enquiry shows, redefinition of the ridge tag because one cannot anymore correctly tag a whole ridge as one way.

We have two options to go from here:
1. Revert that definition change, continue to allow tagging ridges as one way  spanning over several peaks
2. Leave the wiki definition as it is currently and you tag "name=Catskill Divide" on every of the multiple natural=ridge ways you'd have to create along the whole way

I'd favour the first option because
1. Redefinition of an existing tag is a no go
2. The reason why the definition was changed was to make it easier to render a ridge in a certain suggested way. Don't know if it is even rendered this way now (on osm-carto), but in any case this'd be tagging for the renderer, as the information where the ridge goes up and where it goes down is already present in the peak/saddle nodes

Cheers
Tobias

Am 4. Oktober 2018 16:46:19 MESZ schrieb Kevin Kenny <[hidden email]>:

>In some maps that I render, I want to show the divide between a couple
>of major river basins. (I have a good DEM for the area in question and
>can derive the line readily.)
>
>In light of the recent thread on topographic prominence, I wonder if
>this is sufficiently interesting information at least to push it to
>OSM. (If not, that's fine, I have a PostGIS database and a bucket of
>shapefiles and know what to do.)
>
>If it is sufficiently interesting, the question then arises: how to
>map/tag it?
>
>'natural=ridge' comes to mind, and the divide in question has a local
>name. (The 'Catskill Divide' separates the basins of the Hudson and
>Delaware Rivers.) This approach appears to run into problems, as I
>read the Wiki. I see:
>
>> The way should connect saddle points and peaks, and the arrows should
>point upwards.
>
>That may be all right for a ridge ascending the flank of a single
>mountain, but what I'm talking about is the spine of a range, with the
>ridge traversing dozens of named peaks. Even with a single mountain,
>if there are false summits, the arrows on a single way cannot point
>upward all the time! (And the wiki is clear that the
>
>Do I misread, and should the reading instead simply be that the
>arrowhead should be higher than the arrow tail? In that case, I could
>break the divide into two ways, with a common endpoint at the highest
>summit in the range.
>
>Consider this a low-priority item. I have (or will have - there is a
>bit of debugging yet) the data. I know how to render them. I'm happy
>enough with a shapefile or a private PostGIS table if others aren't
>interested.
>
>_______________________________________________
>Tagging mailing list
>[hidden email]
>https://lists.openstreetmap.org/listinfo/tagging

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Re: Map a divide?

Warin
A land form ridge too me is a long, narrow raised part of a high edge formed by hill/mountains and there associated bits.

A land form of a dividing range or continental divide does not have to be narrow, 
The 'dividing line' marks the separate water flow from one side to the other and should be 'long'. 

So I don't think a divide/range is a ridge necessarily. 
In fact a divide could have several ridges as part of the divide. 


Some sample divides/ranges?
Andes 7,000 km https://en.wikipedia.org/wiki/Andes
Rocky Mountains 4,800 km https://en.wikipedia.org/wiki/Rocky_Mountains
Great Dividing Range 3,500 km https://en.wikipedia.org/wiki/Great_Dividing_Range 
The last exists in OSM as an area Way 211234843 tagged as 
fixme=More accurate/better representation/make relation
fuzzy=50000
name=Great Dividing Range
name:cs=Velké předělové pohoří
name:de=Australisches Bergland
name:es=Gran Cordillera Divisoria
name:nl=Groot Australisch Scheidingsgebergte
natural=mountain_range
source=OpenCycleMap terrain rendering
source:name=local_knowledge
wikidata=Q192583

There are 87 natural=divide in the data base ... 
no wiki page to say what it is, looks to be used by North America
Sample found: Relation 1643366 tagged as 
FIXME=continue
name=Sierra Crest
natural=divide
route=natural
type=route

----------------
I do agree that a true ridge line may not point uphill all the time, but still be a single ridge. 

On 05/10/18 01:54, Tobias Zwick wrote:
The definition in the wiki is a bit contradictory, in my opinion. On the one side, it states the thing about that the arrows should point away from the saddle point towards the peaks, like steps or a oneway street, on the other side it describes a ridge to connect several peaks and saddle points. Tagging the whole ridge as one way would be impossible if you followed that "arrow points upward" rule.

There is also no mention of that rule in the original approved proposal. Looking at the history of the article, that rule was added in January 2018, following a short, well "discussion" about how a ridge could be rendered.
The change made amounts to a incompatible, as your enquiry shows, redefinition of the ridge tag because one cannot anymore correctly tag a whole ridge as one way.

We have two options to go from here:
1. Revert that definition change, continue to allow tagging ridges as one way  spanning over several peaks
2. Leave the wiki definition as it is currently and you tag "name=Catskill Divide" on every of the multiple natural=ridge ways you'd have to create along the whole way

I'd favour the first option because 
1. Redefinition of an existing tag is a no go
2. The reason why the definition was changed was to make it easier to render a ridge in a certain suggested way. Don't know if it is even rendered this way now (on osm-carto), but in any case this'd be tagging for the renderer, as the information where the ridge goes up and where it goes down is already present in the peak/saddle nodes

Cheers 
Tobias 

Am 4. Oktober 2018 16:46:19 MESZ schrieb Kevin Kenny [hidden email]:
In some maps that I render, I want to show the divide between a couple
of major river basins. (I have a good DEM for the area in question and
can derive the line readily.)

In light of the recent thread on topographic prominence, I wonder if
this is sufficiently interesting information at least to push it to
OSM. (If not, that's fine, I have a PostGIS database and a bucket of
shapefiles and know what to do.)

If it is sufficiently interesting, the question then arises: how to
map/tag it?

'natural=ridge' comes to mind, and the divide in question has a local
name. (The 'Catskill Divide' separates the basins of the Hudson and
Delaware Rivers.) This approach appears to run into problems, as I
read the Wiki. I see:

The way should connect saddle points and peaks, and the arrows should
point upwards.

That may be all right for a ridge ascending the flank of a single
mountain, but what I'm talking about is the spine of a range, with the
ridge traversing dozens of named peaks. Even with a single mountain,
if there are false summits, the arrows on a single way cannot point
upward all the time! (And the wiki is clear that the

Do I misread, and should the reading instead simply be that the
arrowhead should be higher than the arrow tail? In that case, I could
break the divide into two ways, with a common endpoint at the highest
summit in the range.

Consider this a low-priority item. I have (or will have - there is a
bit of debugging yet) the data. I know how to render them. I'm happy
enough with a shapefile or a private PostGIS table if others aren't
interested.

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Re: Map a divide?

Kevin Kenny-3
On Thu, Oct 4, 2018 at 6:26 PM Warin <[hidden email]> wrote:
>
> A land form ridge too me is a long, narrow raised part of a high edge formed by hill/mountains and there associated bits.
>
> A land form of a dividing range or continental divide does not have to be narrow,
> The 'dividing line' marks the separate water flow from one side to the other and should be 'long'.

That divide is, ipso facto, a ridge line, because water flows downhill
- it is a line that's higher than the basins on either side. It runs
from peak to saddle to peak to saddle (admittedly, the 'peaks' may be
of but little prominence) for the length of the divide.

In any case, the place where I mean to map the divide is terrain of
considerable complexity, topographically. Its high point is the most
topographically prominent feature between Vermont and West Virginia.
The divide unquestionably runs for the most part along high ridges.
The 'ridge' nature gets lost only at a handful of saddles that are in
relatively high-elevation wetlands - but those flat lands are still
some hundreds of metres above the valley floors of the two rivers
whose basins it divides. The complexity comes from the fact that the
place is not a mountain range geologically. It is a 'dissected
plateau', whose 'peaks' are actually arêtes between chasms excavated
by glacial action. The direction of glacial flow was not consistent
among the glacial epochs, so the ridges tend to run higgledy-piggledy,
and one reason for mapping the divide is that it is otherwise tricky
to follow visually. It wanders quite a lot.

I live among some of the oldest exposed rock on the planet -
surrounding me are sediments from the early Palæozoic, and to the
north is the Canadian Shield, with its oldest rock dating to the
Hadean Æon, with its oldest samples having an estimated formation of
4.2 × 10⁹ years ago. Consequently, the mountains here are all rounded
and eroded. The ridges, except for the cols at the boundaries of
glacial cirques, are therefore broader and flatter-topped than you
appear to imagine - but they still rise distinctly above the
surrounding valleys.

Can we agree that https://www.flickr.com/photos/ke9tv/9514469053,
https://www.flickr.com/photos/ke9tv/9764660284,
https://www.flickr.com/photos/ke9tv/10031459724 and
https://www.flickr.com/photos/ke9tv/14799708048 are all indeed views
of a ridge? (In the last photo, ignore the mountains on the horizon,
they're in a different range - the ridge runs diagonally away from the
right foreground).

Or check out the contour lines (elevations in feet; my users are
American) along the Devil's Path
http://kbk.is-a-geek.net/catskills/test4.html?la=42.1294&lo=-74.1438&z=13
and tell me that I'm not dealing with a ridge!

Open question - which I'll resolve for myself - is how much
topographic prominence to use when labeling peaks and saddles.  I
think I'll probably follow the local hiking clubs, and say, '150 feet
(about 45 m) of prominence, and 1 km separation' for mapping peaks and
their key cols, and at this point I care about the key cols mostly so
as to keep the topology of the divide continuous - although I'll
surely map the names of the saddles where I know them! (They do have
local names, even though they aren't listed in GNIS or appear on very
many maps. But the locals could tell you where to find Mink Hollow,
Stony Clove, Lockwood Gap, Winisook Pass or Pecoy Notch - and yes, we
use all those words in toponyms, reflecting the Babel of languages
that our settlers spoke.)

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Re: Map a divide?

Warin
On 05/10/18 09:45, Kevin Kenny wrote:
On Thu, Oct 4, 2018 at 6:26 PM Warin [hidden email] wrote:
A land form ridge too me is a long, narrow raised part of a high edge formed by hill/mountains and there associated bits.

A land form of a dividing range or continental divide does not have to be narrow,
The 'dividing line' marks the separate water flow from one side to the other and should be 'long'.
That divide is, ipso facto, a ridge line, because water flows downhill
- it is a line that's higher than the basins on either side. It runs
from peak to saddle to peak to saddle (admittedly, the 'peaks' may be
of but little prominence) for the length of the divide.

In any case, the place where I mean to map the divide is terrain of
considerable complexity, topographically. Its high point is the most
topographically prominent feature between Vermont and West Virginia.
The divide unquestionably runs for the most part along high ridges.
The 'ridge' nature gets lost only at a handful of saddles that are in
relatively high-elevation wetlands - but those flat lands are still
some hundreds of metres above the valley floors of the two rivers
whose basins it divides. The complexity comes from the fact that the
place is not a mountain range geologically. It is a 'dissected
plateau', whose 'peaks' are actually arêtes between chasms excavated
by glacial action. The direction of glacial flow was not consistent
among the glacial epochs, so the ridges tend to run higgledy-piggledy,
and one reason for mapping the divide is that it is otherwise tricky
to follow visually. It wanders quite a lot.

I live among some of the oldest exposed rock on the planet -
surrounding me are sediments from the early Palæozoic, and to the
north is the Canadian Shield, with its oldest rock dating to the
Hadean Æon, with its oldest samples having an estimated formation of
4.2 × 10⁹ years ago. Consequently, the mountains here are all rounded
and eroded. The ridges, except for the cols at the boundaries of
glacial cirques, are therefore broader and flatter-topped than you
appear to imagine - but they still rise distinctly above the
surrounding valleys.

Can we agree that https://www.flickr.com/photos/ke9tv/9514469053,
https://www.flickr.com/photos/ke9tv/9764660284,
https://www.flickr.com/photos/ke9tv/10031459724 and
https://www.flickr.com/photos/ke9tv/14799708048 are all indeed views
of a ridge? (In the last photo, ignore the mountains on the horizon,
they're in a different range - the ridge runs diagonally away from the
right foreground).

Or check out the contour lines (elevations in feet; my users are
American) along the Devil's Path
http://kbk.is-a-geek.net/catskills/test4.html?la=42.1294&lo=-74.1438&z=13
and tell me that I'm not dealing with a ridge!

You may be dealing with a single ridge line. 
Now tell me there is a single ridge line for the Australian Great Divide and I'd get you to an ophthalmologist.
Some of it is not very sharp, more flat, so not a ridge line. 
There may be cliffs and ridge lines in that area .. but they do not mark the divide. 
 

Open question - which I'll resolve for myself - is how much
topographic prominence to use when labeling peaks and saddles.  I
think I'll probably follow the local hiking clubs, and say, '150 feet
(about 45 m) of prominence, and 1 km separation' for mapping peaks and
their key cols, and at this point I care about the key cols mostly so
as to keep the topology of the divide continuous - although I'll
surely map the names of the saddles where I know them! (They do have
local names, even though they aren't listed in GNIS or appear on very
many maps. But the locals could tell you where to find Mink Hollow,
Stony Clove, Lockwood Gap, Winisook Pass or Pecoy Notch - and yes, we
use all those words in toponyms, reflecting the Babel of languages
that our settlers spoke.)

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Re: Map a divide?

Joseph Eisenberg
In reply to this post by Warin

I do agree that a true ridge line may not point uphill all the time, but still be a single ridge. 
There is also no mention of that rule in the original approved proposal. Looking at the history of the article, that rule was added in January 2018, following a short, well "discussion" about how a ridge could be rendered.
Thank you for noticing this! I had just looked at the proposal page yesterday and was wondering where the directional thing came from.
It will make my life much easier if I can draw a whole named ridge as one way, instead of splitting it at each topographical saddle and peak!
In Northern California, many named ridges are long, but are considered one feature. 
Am 4. Oktober 2018 16:46:19 MESZ schrieb Kevin Kenny [hidden email]:
In some maps that I render, I want to show the divide between a couple
of major river basins. (I have a good DEM for the area in question and
can derive the line readily.)

In light of the recent thread on topographic prominence, I wonder if
this is sufficiently interesting information at least to push it to
OSM. 
Re: the original question, I've been thinking of this myself.

Besides natural=ridge, which is an approved feature, but meant for individual ridges, there is also the proposed feature natural=mountain_range
I believe this is the appropriate tag for the line formed by the crest of the Appalachian Mountains, or the Sierra Nevada and Cascade crests in the USA.
It could either be used as a way that follows the line of each individual ridge, or you could use a relation and add each of the ridges. 

I think natural=divide could be appropriate for a clearly defined drainage divide which is not a mountain range; eg a range of hills or a series of low ridges?
But there's no proposal page or wiki page yet. 

On the previous discussion, several people warned that we should not attempt to map all divides between water drainage areas, because there are some flat areas like plateaus or plains where the drainage is not clear, even if you survey the area in person or check good satellite imagery. But if there is a clear ridgeline, that should be verifiable. I'm not certain of the situation in the Catskills.

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Re: Map a divide?

Joseph Eisenberg
In reply to this post by Kevin Kenny-3


That divide is, ipso facto, a ridge line, because water flows downhill
- it is a line that's higher than the basins on either side. It runs
from peak to saddle to peak to saddle (admittedly, the 'peaks' may be
of but little prominence) for the length of the divide.

Agreed. But I'd keep natural=ridge for individual named ridges (which may have a few named peaks and saddles along it's length)
And use natural=mountain_range for the much larger features which continue several or many named ridges and peaks and saddles.
Or natural=divide
 
Can we agree that https://www.flickr.com/photos/ke9tv/9514469053,
https://www.flickr.com/photos/ke9tv/9764660284,
https://www.flickr.com/photos/ke9tv/10031459724 and
https://www.flickr.com/photos/ke9tv/14799708048 are all indeed views
of a ridge?

Yes, agreed. The page for natural=ridge shows a rather low, gentle hill as an example of a ridge. 

Open question - which I'll resolve for myself - is how much
topographic prominence to use when labeling peaks and saddles.  I
think I'll probably follow the local hiking clubs, and say, '150 feet
(about 45 m) of prominence, and 1 km separation' for mapping peaks and
their key cols, and at this point I care about the key cols mostly so
as to keep the topology of the divide continuous - although I'll
surely map the names of the saddles where I know them! 

I'd map all named peaks that have any topographical prominence, but include the prominence=* in meters.

For example, the most visually significant mountain peak in my hometown is "Lower Devil's Peak";
It rises over 600m from the valley floor, but only has about 15 meters of topographical prominence,
because it connects to a high ridge leading to "Middle Devil's Peak" and "Upper Devil's Peak" (both more prominent).

If you know the topographical prominence it's really helpful to add, to show that this is a minor sub-peak.

For myself, in the USA, I also more or less follow your idea, adding unnamed peaks with over 50m prominence,
especially if they have an accurate spot elevation on the USGS maps, even if they are unnammed.

(BTW, the proposal for "Prominence" is underway; please comment: https://wiki.openstreetmap.org/wiki/Proposed_features/key:prominence)

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Re: Map a divide?

Philip Barnes
In reply to this post by Kevin Kenny-3
You seem to be describing a watershed, which was recently discussed.

https://lists.openstreetmap.org/pipermail/tagging/2018-September/039026.html

Phil (trigpoint)

On 4 October 2018 15:46:19 BST, Kevin Kenny <[hidden email]> wrote:
In some maps that I render, I want to show the divide between a couple
of major river basins. (I have a good DEM for the area in question and
can derive the line readily.)

In light of the recent thread on topographic prominence, I wonder if
this is sufficiently interesting information at least to push it to
OSM. (If not, that's fine, I have a PostGIS database and a bucket of
shapefiles and know what to do.)

If it is sufficiently interesting, the question then arises: how to map/tag it?

'natural=ridge' comes to mind, and the divide in question has a local
name. (The 'Catskill Divide' separates the basins of the Hudson and
Delaware Rivers.) This approach appears to run into problems, as I
read the Wiki. I see:

The way should connect saddle points and peaks, and the arrows should point upwards.

That may be all right for a ridge ascending the flank of a single
mountain, but what I'm talking about is the spine of a range, with the
ridge traversing dozens of named peaks. Even with a single mountain,
if there are false summits, the arrows on a single way cannot point
upward all the time! (And the wiki is clear that the

Do I misread, and should the reading instead simply be that the
arrowhead should be higher than the arrow tail? In that case, I could
break the divide into two ways, with a common endpoint at the highest
summit in the range.

Consider this a low-priority item. I have (or will have - there is a
bit of debugging yet) the data. I know how to render them. I'm happy
enough with a shapefile or a private PostGIS table if others aren't
interested.
Tagging mailing list
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Re: Map a divide?

Warin
I think a 'watershed' is an area describing the catchment area that drains to one point.
A 'divide' is a single line that describes the division between 2 or more watersheds. 

You are right in that they are all water related. 

On 05/10/18 17:20, Philip Barnes wrote:
You seem to be describing a watershed, which was recently discussed.

https://lists.openstreetmap.org/pipermail/tagging/2018-September/039026.html

Phil (trigpoint)

On 4 October 2018 15:46:19 BST, Kevin Kenny [hidden email] wrote:
In some maps that I render, I want to show the divide between a couple
of major river basins. (I have a good DEM for the area in question and
can derive the line readily.)

In light of the recent thread on topographic prominence, I wonder if
this is sufficiently interesting information at least to push it to
OSM. (If not, that's fine, I have a PostGIS database and a bucket of
shapefiles and know what to do.)

If it is sufficiently interesting, the question then arises: how to map/tag it?

'natural=ridge' comes to mind, and the divide in question has a local
name. (The 'Catskill Divide' separates the basins of the Hudson and
Delaware Rivers.) This approach appears to run into problems, as I
read the Wiki. I see:

The way should connect saddle points and peaks, and the arrows should point upwards.
That may be all right for a ridge ascending the flank of a single mountain, but what I'm talking about is the spine of a range, with the ridge traversing dozens of named peaks. Even with a single mountain, if there are false summits, the arrows on a single way cannot point upward all the time! (And the wiki is clear that the Do I misread, and should the reading instead simply be that the arrowhead should be higher than the arrow tail? In that case, I could break the divide into two ways, with a common endpoint at the highest summit in the range. Consider this a low-priority item. I have (or will have - there is a bit of debugging yet) the data. I know how to render them. I'm happy enough with a shapefile or a private PostGIS table if others aren't interested.
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Re: Map a divide?

Joseph Eisenberg
Apparently British English uses "watershed" to name the line that divides to drainage basins, though Americans would call that a "divide":

I looked up natural=divide on Overdrive Turbo. I didn't find any uses in Europe. In North America there were two places where the tag has been used: the continental divide in southern Wyoming, Colorado and New Mexico (not completely finished), and the Sierra Crest along the Sierra Nevada mountain range.

I think the Sierra Crest would make more sense as natural=mountain_range. It's also a drainage divide, but the mountain range is certainly the more significant feature.
The continental divide along the Rocky Mountains does have some sections that are named mountain ranges, but other parts are along low ridges or hills (relative to the surrounding terrain), so perhaps it makes sense ot mark the Continental Divide.

In the previous discussion last month, there were concerns that the watershed boundary or divide would not be possible to determine in some places, for example flat wetlands or plains where there is no obvious ridgeline. Natural=mountain_range doesn't share this problem. But I do think there should be tags for ranges of hills and ridges that don't qualify as a "mountain_range"

On Fri, Oct 5, 2018 at 4:35 PM Warin <[hidden email]> wrote:
I think a 'watershed' is an area describing the catchment area that drains to one point.
A 'divide' is a single line that describes the division between 2 or more watersheds. 

You are right in that they are all water related. 

On 05/10/18 17:20, Philip Barnes wrote:
You seem to be describing a watershed, which was recently discussed.

https://lists.openstreetmap.org/pipermail/tagging/2018-September/039026.html

Phil (trigpoint)

On 4 October 2018 15:46:19 BST, Kevin Kenny [hidden email] wrote:
In some maps that I render, I want to show the divide between a couple
of major river basins. (I have a good DEM for the area in question and
can derive the line readily.)

In light of the recent thread on topographic prominence, I wonder if
this is sufficiently interesting information at least to push it to
OSM. (If not, that's fine, I have a PostGIS database and a bucket of
shapefiles and know what to do.)

If it is sufficiently interesting, the question then arises: how to map/tag it?

'natural=ridge' comes to mind, and the divide in question has a local
name. (The 'Catskill Divide' separates the basins of the Hudson and
Delaware Rivers.) This approach appears to run into problems, as I
read the Wiki. I see:

The way should connect saddle points and peaks, and the arrows should point upwards.
That may be all right for a ridge ascending the flank of a single mountain, but what I'm talking about is the spine of a range, with the ridge traversing dozens of named peaks. Even with a single mountain, if there are false summits, the arrows on a single way cannot point upward all the time! (And the wiki is clear that the Do I misread, and should the reading instead simply be that the arrowhead should be higher than the arrow tail? In that case, I could break the divide into two ways, with a common endpoint at the highest summit in the range. Consider this a low-priority item. I have (or will have - there is a bit of debugging yet) the data. I know how to render them. I'm happy enough with a shapefile or a private PostGIS table if others aren't interested.
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