[OSM-dev] openstreetmapdata.com

Jochen Topf jochen at remote.org
Mon Mar 11 06:41:59 UTC 2013

On Sun, Mar 10, 2013 at 10:33:34PM -0400, Preet wrote:
> First off, thanks to Jochen for creating such an awesome service. The
> hardware I have isn't really capable of extracting coastlines from
> planet.osm in any reasonable amount of time, so having updated data
> available is invaluable for me!
> I had a question about one of the notes on the coastline data:
> "The coastline in OpenStreetMap is often broken. The update process will
> try to repair it, but this does not always work. If the OSM data can't be
> repaired automatically, the data here will not be updated."
> What does this mean exactly? Is there a chance poorly defined coastline
> data won't be seen in the resulting shapefile? ie, could there be chunks of
> coastline simply missing? Or is there a 'decent' baseline, and only newly
> updated coastline data that's bad won't be processed to reflect the changes?

The coastline data is always extracted in its entirety by OSMCoastline.
OpenStreetMapData.com tries to figure out if there are more than trivial
problems with the data and will not publish the data if there are. So you'll
simply get the data from the last run in that case.

This process is not and can not be perfect. If you wait for the coastline
data to be perfect, you can wait for months. Every day errors are fixed but
new errors are introduced. So it can take a long time and many runs of the
OSMCoastline software till you happen to run it at a time when everything
is "perfect". And of course that still doesn't mean the coastline is right,
because we can only check for inconsistencies in the data and not against
the real world.

So what you get is a compromise of sorts. There might be small errors in
the data produced for OpenStreetMapData.com but you can be pretty sure that
there is no continent suddenly missing or so. Before settling on this
compromise I had tried the "wait for everything to be perfect" approach,
but, as I said, it sometimes took months for an update to go through.

While the data update runs on OpenStreetMapData.com it also produces the
coastline view of the OSM Inspector. You can see it at
http://tools.geofabrik.de/osmi/?view=coastline .
If you see no errors there you can be pretty sure there are no errors in
the data at OpenStreetMapData.com.

Jochen Topf  jochen at remote.org  http://www.remote.org/jochen/  +49-721-388298

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