[OSM-dev] osm2pgsql polygon handling semantic?

[Paul Norman]

On 10/19/2015 5:50 AM, Sven Geggus wrote:
> Hello,
>
> looking at the code of osm2pgsql and checking with a hand-crafted .osm file
> I came to the conclusion that polygons in osm2pgsql are handled in the
> following way:
>
> If a polygon is defined in the .style file a function is called which
> generates the simple-feature geometry object.  If this function is unable to
> generate a polygon a linestring is created.
>
> Am I right in this assumption?

For the osm2pgsql pgsql backend, a polygon is formed when a way is 
closed and the tag transform sets a polygon flag. If either of these is 
false, it is a linear feature. For the C tag transforms*, the polygon 
flag is set when there is a tag which has a polygon flag set, or phstore 
which is equivalent to polygon,nocolumn

For the multi-backend, see 
https://github.com/openstreetmap/osm2pgsql/issues/270. This issue also 
contains some information about the pgsql backend.

> The reason for this question is that I'm trying to build a imposm3 style
> which will create a osm2pgsql compatible database.  This has (among other
> things) the obvous advantage, that some additional tables for more exotic
> features are easy to create.

osm2pgsql has the capability to arbitrarily convert tags, for example 
unit conversions, normalize yes/no/unknown values, transliterate values, 
use other PostgreSQL types, and perform other logic. When I looked at 
imposm3, it didn't have the capability to take a script. Has this changed?

Even if not using custom tag transforms, the osm2pgsql C tag transforms 
do perform some tag manipulation.

* They still get called the C tag transforms, even though osm2pgsql is 
C++ now