[OSM-talk] Islands in lakes

[Donald Allwright]

----- Original Message ----
From: Alan Millar <am12 at bolis.com>
To: Donald Allwright <donald_allwright at yahoo.com>; talk at openstreetmap.org
Sent: Thursday, 5 June, 2008 12:28:55 AM
Subject: Re: [OSM-talk] Islands in lakes

>I found a fix for some of the island rendering problems,
>where the island is split across two z12 tiles.  One tile
>looks correct, but the other tile is inverted: it looks like 
>a land area with half a lake when it should look like a water
>area with half an island.

>If the island is tagged with "natural=water", it doesn't work
>right, even when the way is counterclockwise like it should be.
>If you change the island to "natural=coastline", it should then
>render the tile correctly.

There still seems to be something wrong with the tagging around Lake Titicaca - now Mapnik is showing two large tiles of sea, and some of the tiles are still inverted in Osmarender. Not sure if this is a side-effect of the change from natural=water to natural=coastline or some other change.

>What I see in close_areas.pl is that it just looks at natural=coastline
>and not natural=water.

I suspect that rather than changing the tagging to natural=coastline (which feels wrong, as it's not technically coastline) close_areas.pl should be fixed to look for both - I think the previous tagging was correct but the renderer itself was wrong.

There may be a better way of getting this to render correctly - if you look at:
http://openstreetmap.org/?lat=-8.8753&lon=-74.3392&zoom=14&layers=B00FT
I started tracing round a rather spectacularly-shaped lake, but before I got very far I decided to use the opportunity to see of you could separate the lake into separate areas. It seems to render OK in Mapnik and osmarender, which is similar to the way I've been doing the nearby river (to the west). This might turn out to be a simpler way to fix the rendering of Lake Titicaca - if we split it into a number of smaller ajdacent polygons. Each would cover a smaller area (maybe divide it into strips?) and work around the problem of large apparently blank areas. However, there maybe downsides to this - firstly it's another example of changing the data to satisfy the renderers (which is like the tail wagging the dog) and I wonder what effect it will have on (potentially yet-to-be-written) applications using the data. Also, I'm also not sure how to tag multipolygons correctly for this - the lake outer is tagged as a multipolygon with role=outer, the islands
 with role=inner. Dividing the outer into multiple 'outers' could cause a lot of confusion, not least to the renderers themselves.

Cheers,
Donald


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