marco at boeringa.demon.nl
Mon Apr 16 16:34:13 UTC 2018
No, buildings are not the most interesting. I once generalized all buildings in Denmark. It only reduced the storage by maybe 5%, at the high cost of heavily distorting a large number of them. Most buildings in OSM are in fact already in their most generalized state: just 4 nodes. Unless you think triangles is a suitable representation ;-). Besides, buildings are only shown at high zoom, while generalization is most needed and beneficial at low zoom. Lastly, most vector generalization algorithms are primarily designed and effective for rather smooth and node rich data, like a stream-digitized feature, neither of which relates to square buildings. Hence i consider generalizing buildings largely senseless.
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Van: Tomas Straupis
Verstuurd: maandag 16 april 11:48
Onderwerp: Re: [OSM-dev] Generalisation
Aan: Openstreetmap Dev list
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 _______________________________________________ dev mailing list dev at openstreetmap.org https://lists.openstreetmap.org/listinfo/dev
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