[OSM-dev] Generalisation

Tomas Straupis tomasstraupis at gmail.com
Mon Apr 16 09:36:48 UTC 2018


2018-04-16 11:34 GMT+03:00 Martin Koppenhoefer wrote:
> There are some precomputed / extracted data files though, some of which
> contain generalized (simplified) data. These are all "external" sources:
> <...>

  Ok, so this is natural polygon generalisation. Looking at
https://github.com/imagico/coastline_gen the method used is to
rasterise, process and then vectorise back.
  I wonder if that is better/faster than full vector way:
st_clusterwithin, st_union, st_buffer(positiveN),
st_buffer(negativeN+M), st_buffer(positiveM) with a seasoning of
st_simplifypresevetopology according to taste.

> Another aspect is filtering: osm-carto removes features when they would be
> very small (pixels at a given zoom level) and lead to "noise".

  Filtering (selection) is technically also a generalisation.
  But you need to group and probably amalgamate them before deciding
that it is "too small". For example if we have a lot of small patches
of forest close together (say 1000 patches of 10x10m with distance
between patches of 1m) you would want to amalgamate them to one large
forest, not to get rid of them all.

  Ways and especially buildings are the most interesting (difficult) part :-)

P.S. GRASS claims to be doing displacement and way selection
(https://grasswiki.osgeo.org/wiki/V.generalize_tutorial)

-- 
Tomas



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