[Tagging] Additional detail of Levee mapping via embankments

Richard ricoz.osm at gmail.com
Wed Nov 13 21:40:29 UTC 2019

On Wed, Nov 13, 2019 at 06:54:52PM +0900, John Willis via Tagging wrote:

> > On Nov 11, 2019, at 6:15 PM, Martin Koppenhoefer <dieterdreist at gmail.com> wrote:
> > A relation seems easier to evaluate and explicit, while a spatial query heuristic will inevitably fail in some cases
> I think there is a need for a basic relation, if I understand Martin correctly, to simply associate the two lines, (for example, an =embankment and an =embankment_base pair). When mapped, they are not joined. They are merely adjacent. I am not sure of what “type” of relation to choose in iD, but I assume someone will tell us which type to use.
> When mapping a simple cutting or embankment, you would have only one “base” line adjacent - so there is little ambiguity, and the relationship can be inferred (IIUC), but in complicated tagging, there could easily be a situation where which base belongs to which line is unclear, and lead to problems.
> Simply putting them into a relation says “these members are related” and the renderer can know for certain that these two ways that don’t share nodes are a pair, no inference needed. 

perhaps as a last step to perfection we might need some relations. Otoh quite pragmatically 
- what is the use of associating/relating those two lines (base and top)? 
Do we map them to make it clear if you run there you fall down a cliff or earth bank or run
into a cliff? No need for relations for that.

Also I am not convinced there is always a one-one relation between a cliff base and cliff 
If we really wanted to render the "slope/cliff area" in some special style we would probably
have to map that as an area, not relation of two or more lines. But I think for small slopes, 
the top and base lines if rendered should be good enough and for high slopes/cliffs DEM 
derived countour lines would be better.

Btw as we get into more details we might want to map ramps as well.
> This again raises the question of levees - is the levee worthy of it’s own levee relation? do you put all 4 embankment lines into relation with the man_made=dyke line? this seems to be the only solution to:
> - properly group the embankments with the levee
> - not have to use super=relations (putting the embankment relations into a levee relation)
> - providing the most flexibility to weird situations
> - allowing for the extent of the top of the levee to be defined (large levees have varying width tops with usable areas, as shown, in which a “way” is insufficient ). 

We use man_made=bridge (area) to group ways on a bridge.. so I am wondering would 
man_made=levee to encompass the whole levee area work in an equivalent way?
I think it is only a quirk of OSM history that dyke and embankment are linear features 
and we would do many things differently today - and maybe we should do it now.

Also somewhat related, waterway=dam can be either linear (the crown of the dam) or area.
I think we should have one tag for the crown of the dam and one for the area because it
would be often useful to map both of them.


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