[OSM-talk] Islands

[David Groom]

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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