[Tile-serving] [openstreetmap/osm2pgsql] Use libosmium for geometry building (#684)

[Christoph Hormann]

Regarding reprojection and validity - It is probably not a big deal in most applications - normal rendering for example does not really care if the geometry is valid or not but with

* --exclude-invalid-polygon removed. libosmium never creates invalid geometries.

i wanted to point out that this would not imply osm2pgsql never creates invalid geometries with coordinate systems other than geographic.  The most important thing is to make this clear in the documentation.

Ultimately this is a more generic problem anyway, i.e. the need for a reprojection library that is capable of validity preserving reprojection of linestring/polygon geometries.

> joto's area statistics report that approximately 50k out of 242.000k have intersections. @joto tells me that this figure actually includes polygons that are reported multiple times and the real figure is more like 20k self-intersecting polygons of which 6k are relations.

I was talking about visual impact here - large polygons have a much higher likeliness of being broken than small ones - a bit like ceramic components in engineering :smile:.  At the same time large polygons have a high visual impact in map rendering.  The vast majority of polygons in the OSM database are buildings which are obviously rarely broken.

Note i am not against being stricter with polygons in osm2pgsql, on the contrary.  But you need to expect complaints from users because of this.  To get an idea of the visual impact - if you take the difference in Jochen's comparison map with the removal of old style MPs: http://area.jochentopf.com/map/index.html - the impact of removing all invalid polygons osmium currently does not handle is larger than that at z6-8 (that is for water and wood/forest) in many areas, in particular northern Europe.


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