[OSM-talk] Islands
Artem Pavlenko
artem at mapnik.org
Sat Apr 14 11:02:39 BST 2007
On 14 Apr 2007, at 10:47, 80n wrote:
> David
> I think there are a couple of problems with closing off the ends of
> the river sections.
>
> Firstly, purists would argue that a segment crossing the river is
> an artifice that does not represent anything in the real world.
>
> Second, you would need two segments, or the segment needs to belong
> to two ways (the upstream way and the downstream way). If it
> belongs to two ways then for one of them it is pointing in the
> wrong direction. (segment sorting will fix that, but that requires
> running frollo before Osmarender).
>
> I've contemplated doing this myself several times and always
> managed to persuade myself not to.
>
> 80n
80n,
We don't really need all this crossing segments.
Please, could you explain to me what's wrong with representing
features like 'river with islands' as proper polygons (one exterior
and n-interior rings) ? If you want just a small part of a bigger
polygon for rendering use _clipping_.
Cheers,
Artem
>
> BTW I noticed an error on the Baghdad map. The al-Sarafiya bridge
> is still shown, but it was blown up last week ;)
>
> On 4/14/07, David Groom <reviews at pacific-rim.net> wrote:
> If the ends of the riverbank were closed off with segments so the
> riverbank formed a complete polygon, would the method outlined
> below by 80n be enough to render rivers and islands correctly in
> the mapnik layer?
>
> David
> ----- Original Message -----
> From: 80n
> To: David Groom
> Cc: talk at openstreetmap.org
> Sent: Saturday, April 14, 2007 9:21 AM
> Subject: Re: [OSM-talk] Islands
>
> 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
> 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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Artem Pavlenko
http://mapnik.org
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