# [OSM-talk] Ditches

Steve Bennett stevagewp at gmail.com
Tue Dec 15 11:18:24 GMT 2009

```On Tue, Dec 15, 2009 at 8:07 PM, Mike Harris <mikh43 at googlemail.com> wrote:
>> Layers are only there to explain the relative heights of
>> things when they meet. No harm will result from marking the
>> ditch as layer -1.
>
> See my separate reply - I disagree - what happens when the "level=-1" ditch
> runs downstream into a "level=0" stream / river - without a waterfall?

Asbolutely nothing. You're waaaay overthinking this, both of you.
Layers are just a hack to make stuff render. It's not like
"bicycle=no" or something where we're making some statement of fact
about the real world. Layers are *not* a statement of fact. Layer=3
does not, in the absolute, mean anything different from Layer=2.

>
>> Whether or not it needs to be a lower number than that of the
>> bridge is an unresolved question.
>
> I disagree - surely the bridge is above the water in the ditch and so - by
> your own defintion ('relative heights') it must have a higher level value?

You're trying to apply some sort of intuition or logic to this. Don't.
It's not some logic puzzle where the layers all have to mean
something. I've worked in areas where someone, for some reason, has
tagged all the bike paths in a park as layer=1. It didn't matter. I
eventually deleted the layer tags because they interfered with my own
tagging scheme, but it was nothing more than personal preference.

>> Not sure I'd even mark it "barrier=ditch" after all that. I'd
>> also only specify a layer for the bridge, not the ditch/drain.
>
> Agree - enough to mark it as a stream or, if that is felt to be too 'big'
> then waterway=ditch.

I doublechecked the wiki, looks like "barrier=ditch, waterway=drain"
might be the right way to go. Belt and braces, you know.

> Also agree that the bridge, rather than the ditch, should carry the layer
> tag (see my comment above).

It. Really. Doesn't. Matter. :)

Say you have a stream at layer=3, and somewhere else it crosses a big
complicated bridge which for some reason someone has tagged layer=-2.
You know what you do? You don't panic. You break the stream, you set
the new part as layer=-3, and you carry on.

>Doesn't this rather imply that the ditch has the
> same layer value as the level=0 surroundings (as I suggest) rather than
> level=-1 (as per your 'no harm' suggestion) - and that the bridge has a
> layer value higher than 0, so presumably level=1 (as I suggest)?

Overthinking.

I am curious to know if any routers look at layers when you have
something like a big routable area (eg, highway=pedestrian) with
barriers within it, though.

Steve

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