[Talk-GB] Thrapston viaduct

SK53 sk53.osm at gmail.com
Tue Jul 14 11:36:14 UTC 2015


Actually creating built-up areas from OSM data is fraught with problems:
the basic one, being that OSM landuse/landcover is too fine-grained for
identifying built-up areas. This is one of the examples in my category of
'emergent data': data which is sort of there, but is actually quite hard to
create from the way it's mapped in OSM at the moment. Generally, I believe
if people want such data they should add it to OSM, rather than expect
people to do complex data wrangling which may not work: an analogy might be
streets mapped as areas, the ways are still needed.

Obviously there are other issues::

   - Incomplete data (easy to fix, map it).
   - Landuse categories which are typically urban in non-urban situations
   (various kinds of industrial, extensive grounds of private schools etc).
   - Little gaps in well-mapped urban fabric (could be filled by a step of
   positive & negative buffering).

The Heidelberg folk tried using data mining (Rapid Miner) to identify urban
areas some time ago: clusters of residential roads, lower speed limits,
lit=yes, sidewalk=both, and probably a few others are the types of tags
which could be used.

In many ways built-up areas are actually more useful than many of the
landuse/landcover categories we already use : at least for more general
applications, such as the more sophisticated cartography discussed here.

Jerry

On 14 July 2015 at 09:36, Richard Fairhurst <richard at systemed.net> wrote:

> Dan S wrote:
> > Sounds good. Mind if I ask how it is done? (i.e. rendering rules
> > for rural vs town)
>
> Post-import, I run a couple of queries along the lines of
>
> UPDATE planet_osm_point SET urban=true FROM built_up_areas WHERE
> ST_Contains(built_up_areas.geom,way)
>
> using a pre-existing 'built_up_areas' table which contains polygons of,
> well, built-up areas. I use OS Open Data for the polygons but you could no
> doubt construct them from OSM landuse if that floats your boat.
>
> The Mapnik stylesheet queries then simply respond to that column:
>
> #poi[type='pub'][urban=true][zoom>=15],
> #poi[type='pub'][urban=false][zoom>=13] {
>   ...rendering rules...
> }
>
> cheers
> Richard
>
>
>
>
> --
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>
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