[Tagging] Culverts and Fords

Volker Schmidt voschix at gmail.com
Fri Mar 2 15:17:51 UTC 2018


The layers tag in OSM is only to enable the renderer to display/draw
crossing OSM elements correctly. The element with the higher layer value is
drown over the ones with lower layer values.

In the case of the ford the waterway and the highway are on the same layer
and share a node, which represents the ford (assuming that highway and
waterway are line objects. If the waterway and/or the highway are drawn as
polygons, then the ford becomes a line or a polygon.)

In case of a culvert the objects are not on the same layer. The highway is
above the waterway (which may be intermittent or a wadi). Hence a common
node is not correct. You could argue that the culvert becomes a node on the
lower way, that is not connected with the highway above. It would have to
be drawn exactly on the crossing point, without being part of the highway
above. A non-zero length culvert is most likely easier to draw, and also
closer to reality.
Again, when the waterway is drawn as a Polygon, the culvert becomes a
polygon.

On 2 March 2018 at 15:41, Vao Matua <vaomatua at gmail.com> wrote:

> Thank you Ralph, I understand your perspective, but have to disagree a bit
> (I'm not looking for a battle, however).
>
> A ford is a stack of layers that are directly adjacent vertically, with
> the road slightly below the stream/river.  In the dry season a ford is only
> a road and only becomes a ford when a watercourse flows over the top of the
> road.
>
> A culvert is a part of of road construction, a culvert would not exist
> without the road, but the culvert is utilized by the stream.  Personally I
> have physically installed culverts in road profiles where there is no
> watercourse.  If I try to add a culvert in JOSM without an additional tag I
> get a validation warning.
>
> Wouldn't a road/stream crossing without a culvert or bridge be called a
> dam?
>
> Isn't a culvert similar in rendering to an embankment?  An embankment is a
> tag applied to a road or railroad, but it is a level beneath the road or
> railroad.  A culvert happens to be perpendicular or so to the road rather
> than adjacent to it.
>
> Part of this discussion also is a matter of scale.  At some rendering of a
> map even a place like Paris would be displayed as a node.  In the same way
> a culvert displayed as a node would be appropriate at certain zoom levels.
>
> I think an easy solution is to make the rendering rule for culverts be a
> layer below the road and allowed to be a node.
>
> I think this is an interesting discussion and is helping me understand
> different points of view, thanks.
>
> Emmor
>
> On Fri, Mar 2, 2018 at 12:39 AM, Ralph Aytoun <ralph.aytoun at ntlworld.com>
> wrote:
>
>> The real easy way to understand *culverts* and *fords* for OpenStreetMap
>> is about the layers they are on and this dictates the nodes they use.
>>
>>
>>
>> For a  *ford* the stream/river is at the same level as the road
>> (effectively *layer=0*) and therefore they are able to share a node.
>>
>>
>>
>> Because a culvert (*layer=-1*)  is not on the same level as the road but
>> passes underneath so it cannot share a node with the road and therefore the
>> culvert is attributed to the river/stream with a node either side of the
>> road.
>>
>>
>>
>> With a *bridge* the road (*layer 1*)  is not on the same level with the
>> stream/river so again cannot share a node and therefore the bridge is
>> attributed to the road with a node at each end of the bridge.
>>
>>
>>
>> Hope this will be of help in understanding the problem.
>>
>>
>>
>>
>>
>> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
>> Windows 10
>>
>>
>>
>
>
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>
>
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