Talk:Proposed features/Waterways classification

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Comment from Carnildo

None of the proposed methods of classifying waterways seems appropriate for OSM:

  • "Classic" can be determined from an incomplete data set, which makes it possible to add it to OSM. But by the same token, an end-user can easily compute it from the OSM dataset, which means we shouldn't include it. (We don't include things like the lengths of roads or the areas of farms for exactly this reason.)
  • "Strahler" and "Shreve" require a complete set of a river's tributaries to compute. The vast majority of OSM's river networks are incomplete, so unless a mapper has an ODbL-compatible outside source, they can't add correct information. In the cases where the network is complete, the same objection to the "classic" ordering applies.

Yes, we need a way of differentiating the Mississippi River from the Elm River, or distinguishing a stream you can cross without breaking stride from one where you're going to get your feet wet, but this isn't it. Posted by User icon 2.svgFanfouer (on osm, edits, contrib, heatmap) on behalf of System-users-3.svgCarnildo (on osm, edits, contrib, heatmap) message on @tagging mailing-list.

Thanks for your comment (and for posting it here). I hope you can get through the captcha, because otherwise it will be hard to discuss.
1. I doubt that computing would be "easy", but it's better if we check it. We can compute many things, like centroid of shape, but for example city labels are sometimes placed manually (like Sidney) for some reason (because it's more predictible, precise etc.). I bet it's not always possible to tell if the river goes to the sea through a bay or lake and human decision would be needed.
2. None of this method require complete data set. In classic you can start with rivers going to the sea, with Strahler/Shreve you can start with the sources. This is still a data that one can use.
3. For some of them we already know the numbers and guess what happens if somebody comes with such complete data set for one river she knows well? She will create ad hoc tags, other than somebody else, so we will have no way to compare them all. That's why having a tagging scheme is useful even for optional data.
--Kocio (talk) 14:50, 14 August 2017 (UTC)

Computable

Why require mappers to calculate a river order, while this information can be added with a script on a PostGIS database? I'd prefer something subjective, like splitting "river" into three subtypes. Like river=small, river=big and river=major. --Zverik (talk) 10:33, 14 August 2017 (UTC)

1. I believe that it's not that easy, but would be happy if somebody make an actual implementation of it to test our intuitions. In case of osm-carto, we have lua preprocessing available, but I'm not a programmer.
2. The hardest part is defining what is small/medium/big river. That's why I gave an example so we could start with something reasonable, but there are many people who require to have objective criteria, so we might end up discussing it for years and see no gain.
--Kocio (talk) 10:52, 14 August 2017 (UTC)
I'll answer 2, since 1 would require a better programmer, like User:Komяpa. We can start with a precise numeric limits for these, ranking by length (e.g. 200 and 1500 km) and width (e.g 10 km from the estuary, 100 m and 1 km). Mappers haven't complained about such limits for setting a place=* tag. --Zverik (talk) 11:23, 14 August 2017 (UTC)
This is when using the complex measures gets tricky:
1. What if river has medium length, like 1000, but big width, like 15 km (or the other way around), how should we decide then? Maybe it's logical OR (any value that meets this limit)?
2. People expect these values to be somehow significant. I was proposing to define a hill with something like max 300 m, and the response was - "why 300 m limit?"
Roads classification was done so early, that we just accept it, as highly subjective and complex as it is, and make agreements how these rules apply to local road system (see my analysis of primary road definition). --Kocio (talk) 12:56, 14 August 2017 (UTC)
It is to be decided. I'm suggesting a general way of tagging, and we can refine it in a proposal. Regarding significance, everyone was okay with place types defined as round numbers (see history). I chose 200 km because of some rivers I know, and 1500 km after reading wikipedia:List of rivers by length. --Zverik (talk) 14:19, 14 August 2017 (UTC)
I'm ready to take part in developing your proposition in parallel to this one. Could you start with a draft, so we could discuss details further? --Kocio (talk) 14:23, 14 August 2017 (UTC)
I can, but next week: got to prepare for SotM now. --Zverik (talk) 14:39, 14 August 2017 (UTC)
Great, it will still be as important by then as now. =} You can also discuss it there to know what people think about it. --Kocio (talk) 14:42, 14 August 2017 (UTC)

Width vs Stream order

For comparison: Annual discharge of US rivers

I fully support the idea, that a more fine-grained waterway classification system is needed. But I don't think, that the stream order is well-suited for this task.

For example the Missouri River has a Strahler order of 9 and the Ohio River just 8 ([1]), despite the Ohio has a much higher average discharge than the Missouri (7,957 m3/s vs 2,478 m3/s). And the Ohio River is even wider than the Mississippi at their confluence!

I'm not sure, but aren't rivers in mountaineous rivers with a dendritic river network quicklier getting higher stream orders than rivers originating in lowlands with less confluences?--Ethylisocyanat (talk) 18:22, 18 August 2017 (UTC)

All of these systems are used for different things. Looking at the definitions, Shreve is better correlated to discharge volumes and Strahler is meant for other uses. It's good to have more of them to cover more needs.
But the fact is we don't know too much yet and I hope we'll try different approaches, including computing order by software. --Kocio (talk) 21:57, 18 August 2017 (UTC)
@Kocio It would be great to have a map with annotated stream orders as example. I've tried with NHDplus, but wasn't able to process it. Do you have any example ready?--Ethylisocyanat (talk) 12:08, 19 August 2017 (UTC)
That would be great, indeed, but I'm not aware of such map. --Kocio (talk) 21:27, 19 August 2017 (UTC)