[Tagging] Additional detail of Levee mapping via embankments

[Joseph Eisenberg]

> They usually have a 2-10m wide “top” on the levee

The tag man_made=embankment should always be placed at the top of the
embankment, so your two lines will only be 2 to 10 meters apart. This
tag is not meant to show the size of the embankment or levee, but the
location of the top of the steep slope. In this usage it is similar to
natural=cliff and barrier=retaining_wall

"it should be tagged on a way drawn with the lower side on right side
of way direction" - Tag:man_made=embankment

If you map the levee as man_made=dyke you can add width=* to each
segment to show how wide it is; this is much faster for mapping, and a
well-designed renderer should be able to show these nicely.

If anyone really wants to map the whole area of the levee, I would
suggest a new tag like area:man_made=dyke - but I think this is not a
good use of mapper time.

If the levee ("dyke") is very large, it should be visible clearly in a
DEM (digital elevation model), like natural hills, slopes and cliffs.
We do not map elevation contours or data in OSM, because a
high-resolution DEM works very well of this, but a vector database
does not.

- Joseph Eisenberg

On 11/11/19, John Willis via Tagging <tagging at openstreetmap.org> wrote:
> As related to my other posts, I am mapping large water containment
> features.
>
> When I began mapping, I often mapped embankments & retaining walls used for
> roads and infrastructure, and during that time, the embankment tag evolved
> to support two embankment lines that would denote the top and bottom of the
> extent of the embankments. This was perfect for me, as there are many
> tollways that sit on a large man-made embankments as they cut trough the
> countryside. Most tollways in Japan are elevated on fill to make crossings
> (via tunnels) much easier, as they cross so many existing small roads.
> mapping the extent of the embankments clearly shows the footprint of the
> tollways through the countryside - much greater than any trunk road.
>
> https://www.openstreetmap.org/#map=17/36.33635/139.40197
> <https://www.openstreetmap.org/#map=17/36.33635/139.40197> - Kita-Kanto
> expressway near Ota-Kiryu IC & Watarase River
>
>
> As I mapped the embankments, I started mapping the levee embankments as
> well, as they are not uniform in shape, with natural and man-made features
> making their shape highly irregular on both the top and bottom, the two sets
> of embankments easily outlining these huge features (usually between 6-12m
> tall and 20-60m wide). They usually have a 2-10m wide “top” on the levee.
> They similarly have a huge footprint compared to other features.
>
> Recently, I realized there is a man_made=dyke tag that is supposed to map
> the “top” of the levee, but there is no documented way to map the *extent*
> of these large flood control features, which feels incorrect.
>
> https://www.openstreetmap.org/#map=17/36.23909/139.68483
> <https://www.openstreetmap.org/#map=17/36.23909/139.68483> - the extent of
> these levees is much greater than the cycling roads on top.
>
> I am going to continue to map the extent of these large man-made levee
> embankments as 2 pairs of embankment lines, and I'll now go back and map the
> levee top with a man_made=dyke line, denoting the “levee route”. I’m
> guessing there are 500km of these large levees in the greater Tokyo area
> alone, with more than a thousand km of somewhat smaller ones.
>
> The levees follow the river through open plains, but their route often is
> constrained occasionally by natural features, where the outer-side of the
> levee is a natural rise for a short distance, but the inner-side is still a
> continuous man-made embankment. being able to separate the almost always
> continuous levee from the extent of it’s two embankments (which merge,
> separate, appear, and disappear) is very useful.
>
> https://www.openstreetmap.org/#map=17/36.23164/139.31544
> <https://www.openstreetmap.org/#map=17/36.23164/139.31544> - levees meet and
> end as one river joins another. Their size varies greatly, denoted by the
> embankment lines.
>
> I feel this should be accepted mapping for extremely large levees, such as
> the ones I am dealing with, where the =dyke way cannot properly express the
> extent of the levee’s breadth and complexity, and the “Top” of the levee is
> not always the center of the structure.
>
> Is it useful to turn this into a relation? with levee embankment members
> being inner-bottom, inner-top, outer-bottom, outer-top and the man_made=dyke
> member  being the “route" of the levee? Maybe it isn’t important to relate
> them. I don’t know.
>
> Thoughts?
>
> Javbw
>
>