----- Original Message ----
From: Alan Millar <[EMAIL PROTECTED]>
To: Donald Allwright <[EMAIL PROTECTED]>; [email protected]
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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