[talk-ph] better ways to coordinate coastline mapping?

Eugene Alvin Villar seav80 at gmail.com
Thu Jul 15 15:06:24 BST 2010


Here's a little bit more elaboration on the problem of coastlines in Mapnik.
This information can be found at the OSM Wiki:
http://wiki.openstreetmap.org/wiki/Coastline

Unlike every other feature in the OSM database, coastlines are rendered
differently in Mapnik. The Mapnik rendering software needs closed polygons
to render areas, and since coastlines are the largest and most basic
polygons there is on any map, OSM has to ensure that it feeds Mapnik closed
coastlines in order to coherently show seas and islands.

For zoom levels 0-9, OSM ignores all coaslines data in OSM and uses the
public domain VMAP0 data. For zoom levels higher than 9, OSM uses the
Coastline error checker tool <
http://wiki.openstreetmap.org/wiki/Coastline_error_checker> to generate
closed shapefiles that Mapnik can use. This tool is only run infrequently
and that is why coastlines do not update often in Mapnik.


On Thu, Jul 15, 2010 at 1:48 PM, maning sambale
<emmanuel.sambale at gmail.com>wrote:

> On Thu, Jul 15, 2010 at 1:41 PM, Jim Morgan <jim at datalude.com> wrote:
> > maning sambale wrote, On Thursday, 15 July, 2010 12:01 PM:
> >> What I mean here is the mainland of Palawan.  But yes the smaller
> >> islands needs more work.
> >
> > Hmmm, now I'm beginning to think I didn't understand. I thought you were
> saying that the coastline of Palawan (yes, the main island) was 99% free of
> jagged coastlines. But when I look at it I can see about 30% unjagged, and
> 70% jagged. eg
> >
> > <
> http://www.openstreetmap.org/?lat=9.1627&lon=118.2309&zoom=13&layers=B000FTF
> >
> >
> > Am I looking at the wrong thing here? Do you have a different view with
> all the jaggies removed? Ah ... or maybe I'm looking at the OSM rendered
> map, which hasn't been updated. Is that it? In which case, sorry to be dumb.
> Try the osmarender layer
>
> http://osm.org/go/4nECSgr?layers=0B00FTF
>
> Mapnik coastlines doesn't update too often.
>
>
> > Jim
> >
> > --
> >   datalude: information security
> >   e: jim at datalude.com
> >   Philippines: +63 2 403 1311 / mob: +63 920 912 5830
> >   Hong Kong: +852 6840 6693
> >   w: http://www.datalude.com/
> >
>
>
>
> --
> cheers,
> maning
>
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