[OSM-talk] hierarchical or flat multipolygons?

[malenki]

On 23 Dec 2014 04:19:49 -0000,
Russ Nelson wrote:

> Hi. I'm editing the Hudson River all as one entity, from its beginning
> in the Adirondacks to its end at NYC. Along the way, there are some
> islands that need to be excluded, so it's not just riverbank,
> riverbank, riverank. It's riverbank, multipolygon(riverbank, island,
> island), riverbank.
> 
> In theory at least, I could make the whole riverbank into one
> multipolygon, with a bunch of outers (about 50) and inners (about
> 200). Would that create a problem for other editors? To load the whole
> thing, you would have to load a LOT of nodes.

That would not only be a problem for other editors but create a
relation which is hard to handle, easily to break and sooner or later
/will/ be broken more or less accidental by someone.
See also http://wiki.openstreetmap.org/wiki/Relation#Size

> Or, conversely, should I keep it as about 20 plain ways and 30
> multipolygons? But what kind of a relation do I make for the entire
> riverbank? 
>[…]
> Should this "Hudson Riverbank" relation be flat or hierarchical?

In the early beginnings I also used to put all stuff for one river in
one relation – only to rip it apart later after gathering enough
knowledge.

Though there is a relation type waterway¹ I think it and all the work
some people put in it superfluous – even more with the not
really documented watershed relation² which mappers want to show a whole
watershed… 
See also:
http://wiki.openstreetmap.org/wiki/Relations/Relations_are_not_Categories

When you have a highway split into several parts because of varying
surface, maxspeed, sidewalk values etc you usually don't create a
relation with the name of the highway. The same logic you can apply for
rivers and watersheds. When all the parts of the riverbanks have
riverbank=* and name=* on them it is an easy task to query for them,
e.g. with http://overpass-turbo.eu

Regarding watershed relations (although you didn't ask for them):
Martin Kompf managed to make a map of all waterways of Central Europe
showing the watersheds – without having relations everywhere:
http://www.kompf.de/gps/rivermap.html

tl;dr regarding your question 
> Should this "Hudson Riverbank" relation be flat or hierarchical?

If you still think you need a relation for all Hudson River, don't make
it a flat one – this would be the worst choice.

hth
Thomas

¹ http://wiki.openstreetmap.org/wiki/Types_of_relation
² http://wiki.openstreetmap.org/wiki/Relation:watershed