[Tagging] Additional detail of Levee mapping via embankments

John Willis johnw at mac.com
Wed Nov 13 09:54:52 UTC 2019

(I mis-sent this email)

> On Nov 13, 2019, at 3:44 AM, Richard <ricoz.osm at gmail.com> wrote:
> We need new tags for the bottom of embankmets, top of cuttings, bottom of cliffs, earth_banks 
> and maybe a few others if we want to map them.

that is very true. 

I think we can cleanly do this with the ways you mentioned. 

We need to chose a scheme for these “base” tags that doesn’t reinvent the wheel, isn’t vague, and can easily be interpreted. my mis-reading of embankment led to some big problems, simply because I assumed the tag could document things it couldn’t.  Your tags look really good to me.

> 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. 

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 ). 

But I am unsure that this is the “only way” and perhaps putting the two embankment relations + dyke line into a levee super-relation would allow mapping of the embankments to be a uniform process (making mapping the details of levees a bit more complicated at the expense of standardized embankment mapping). 

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