[Tagging] Map a divide?

Tobias Zwick osm at westnordost.de
Thu Oct 4 15:54:27 UTC 2018

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


Am 4. Oktober 2018 16:46:19 MESZ schrieb Kevin Kenny <kevin.b.kenny at gmail.com>:
>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
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