[Tagging] Map a divide?

Joseph Eisenberg joseph.eisenberg at gmail.com
Fri Oct 5 09:47:14 UTC 2018

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

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 <61sundowner at gmail.com> 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 <kevin.b.kenny at gmail.com>
> <kevin.b.kenny at gmail.com> 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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