[OSM-dev] OSM based replacement for builtup_area.shp in standard style

[Christoph Hormann]

On Tuesday 03 June 2014, Martin Koppenhoefer wrote:
>
> As a sidenote I wanted to point out that there are currently already
> 281547 ways (I suspect most of them to represent areas) tagged with
> place=* (not much compared to a total of 3.1M places in OSM) which
> could serve as an alternative to your (preprocessing intensive)
> process if more mappers could be convinced for this concept of
> mapping settlement extensions.

Note many of those are islands, see:

http://taginfo.openstreetmap.org/keys/place?filter=ways#values

And the others - like Frederik said - are most likely not really 
describing the actual builtup area.

> The main problems of your approach --- it is still only "guessing"
> and the derived dataset might have a high probability to identify
> "built-up" areas but it will not in all cases be able to tell which
> settlement an area belongs to and it is a ressource intensive process
> that reasonably cannot be done on the fly --- could be overcome with
> explicit place polygons. On the downside it will of course take us
> quite some time to manually map all those settlements, and it is not
> even clear if people are interested in general to map this kind of
> feature (in addition to landuse etc.). If the main style picked this
> up and rendered areas with
> place=(city,town,village,hamlet,isolated_dwelling) as "built-up-area"
> this would surely help promoting the cause ;-)

I think i'd disagree here.  place=city|town|... is simply not a well 
defined area.  You might think you can draw a line around any 
settlement in the world but there is no way to say where exactly this 
line is to be in a verifiable way.  So if you start mapping this 
everyone will draw it differently and nothing is gained in the end.

What i do is essentially defining the builtup area as the area where the 
density of roads and buildings exceeds a certain limit.  This in itself 
is no guesswork at all - the base data is of course very variable and 
there are a lot of subjective choices involved in the process that 
follows.  You might feel inclined to adapt this density measurement as 
a rule for manually mapping areas with place=* but keep in mind 
measuring object density is always specific to the scale you look at 
and there is no natural scale of settlements you can base this on.

-- 
Christoph Hormann
http://www.imagico.de/