[Tagging] Rivers classification

Daniel Koć daniel at xn--ko-wla.pl
Sun Aug 6 18:06:35 UTC 2017

W dniu 06.08.2017 o 19:07, Lukas Sommer pisze:

[You have replied to me personally, but I guess it should go to the list]

> I would encourage to use only _one_ classification system. Otherwise,
> the tagging will be _very_ complex and using it for rendering nearly
> to impossible, because it will be impossible to compare between
> different classification systems.

3 main systems are used in cartography for different things (see the 
Wikipedia article):

- [order:classic] is suitable for general cartographic purposes [...]. 
The first order stream is the one which, at each confluence, is the one 
with the greatest volumetric flow, which usually reflects the 
long-standing naming of rivers.

- [order:strahler] is designed for the morphology of a catchment and 
forms the basis of important hydrographical indicators of its structure, 
such as bifurcation ratio, drainage density and frequency.

- [order:shreve] is preferred in hydrodynamics: it sums the number of 
sources in each catchment above a stream gauge or outflow, and 
correlates roughly to the discharge volumes and pollution levels.

Adding optional tag does not make anything hard, it just allows to use 
our database for more purposes, including hydrographic maps and 
analysis. I don't think we should encourage any of them, because there's 
no "the best" classification system.

> It seems to me that “stream order” means basically “level of
> branching”. This information is almost useless for rendering at
> openstreetmap-carto, because you can have a high level of branching
> even on very small rivers, and you can also have a very low level of
> branching on very large rivers. What’s interesting for rendering is in
> my opinion rather the width of a river. The key “CEMT” seems to me the
> better choise here (but seems to be europe-centric), but also the
> “width” key might help and is likely to be found all over the world.

For general rendering on low zoom I would probably use combination of 
classic and Strahler/Shreve.
order:classic=1 means only that this river goes to the sea, which is 
more important than the river with higher number, but 
order:strahler/order:shreve=1 means also that it's not important river 

"Like a halo in reverse" [M. Gore]

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