[OSM-talk] Islands

[80n]

One method that currently works is to tag islands as part of the same way as
the riverbanks, but make them go anti-clockwise.

So you have one way that goes up the left bank of the river for, say, 1km
then goes down the right bank for for the same distance.  If the two
riverbanks are imagined to be part of a closed way then the segments should
point in a clockwise direction. Then all of the islands within that 1km
section are also made part of that same way, but tagged in an anti-clockwise
direction.

This seems to work pretty well, even when cut at arbitrary points by a bbox,
and is understood by SVG so renders with Osmarender.  The same principle
also workd for islands in lakes and any other situation where something has
a "hole" in it.

There are some examples along the River Thames:

Simple example:
http://www.informationfreeway.org/?lat=6698535.88756&lon=-57979.86205&zoom=16&layers=B000
Complex example:
http://www.informationfreeway.org/?lat=6688981.2611&lon=-51110.08562&zoom=16&layers=B000

80n

On 4/14/07, David Groom <reviews at pacific-rim.net> wrote:
>
>  I don't know :)
>
> I've been meaning to update the large rivers page [1] to get debate going
> on the whole large rivers  / lakes idea, which would include a discussion on
> how to render islands.
>
> Straying away from your initial question, and looking at large rivers,
> Osmarender currently renders large rivers, but the Mapnik layer does not.
>
> As I understand it Mapnik needs a proper polygon to generate the fill and
> so does not generate a filled wide river from the proposed rendering on the
> large rivers [1] page. Own its own this is fairly easy to implement.
>
> Moving now to your question about islands :
>
> It is, as far as I can see, fairly difficult to intuitively split a river
> with islands in it into polygons (particularly where there are many islands
> close to each other) , where each polygon would be a continuous way and
> there is not a complicated system of segments going between the riverbank
> and each island.
>
> I'd like to see a modification of my proposal on the large rivers page [1]
> where riverbanks would be drawn as a polygon with a tag something like
> waterway = riverbank, and islands in these rivers would be drawn as
> waterway = riverbank; island = yes.  Given this suggested rendering, island
> in lakes could be tagged exactly the same as islands in rivers and they
> would still render correctly.
>
> David
>
>
> [1]
> http://wiki.openstreetmap.org/index.php/Proposed_features/Large_rivers
>
>
>
> ----- Original Message -----
> *From:* lewispusey <lewispusey at earthlink.net>
> *To:* talk at openstreetmap.org
> *Sent:* Friday, April 13, 2007 5:08 PM
> *Subject:* [OSM-talk] Islands
>
> What is the current best way to draw and tag islands in rivers and lakes?
> Lewis
>
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