How to select specific role members of relations and combine to create geometry

[mboeringa]

Hi @joto,

We had the opportunity to talk about some of the new functionality and developments regarding osm2pgsql at SOTM 2022 (really enjoyed the conference!), and my specific question whether it was now possible to select specific role members of a relation, and use only those for creating a geometry in flex output.

The specific use case being, as explained then and before, the ability to select only those members with the role 'main_stream' of a 'waterway' relation.

You suggested I should look at the new options to create a geometry collection, and then go over the members of the collection to sort things out in Lua.

However, reading quite extensively through the updated osm2pgsql manual, it is still unclear how this would help my use case, as the geometry collection only contains the actual geometries, but no information about e.g. the roles, and there appears no clear way to store or access this vital information.

You already suggested there was not yet a straightforward way to do this. What we actually appear to need is some kind of filter statement, most likely as an input parameter to the

osm2pgsql.process_relation(object)

function, so that it becomes possible to filter out certain roles, although this would also require the ability to specify relation type specific filter statements, e.g. as a Lua table, as a single filter statement should only be applied to a single specific relation type, as with the 'main_stream' role for the 'waterway' relation type.

Jochen, what are your thoughts on this? And if you still see an option to do this with the current v1.7.0 version of osm2pgsql, could you perhaps give some more clues as to how, or include a working example in the repository's 'flex-config' example styles folder?

Actually, interestingly, I noticed the old pgsql output option, actually appears to have a function that gives access to some of these properties of a relation, like the roles:

filter_tags_relation_member(tags, member_tags, roles, num_members)

Don't know how this fits in the bigger picture of the new flex output though, maybe you have something sensible to say about that, although it is clear from the documentation this function only concerns tags, not geometry processing.

[joto]

In https://osm2pgsql.org/doc/manual.html#processing-callbacks you can see that there is a members array you can access from the parameter of the processing callback which contains all the members with their roles. The GeometryCollection we spoke about contains one geometry for each node and way member, so you can iterate over both the object.members array as well as the result of object:as_geometrycollection() at the same time, ignore the members of type relation and you have a one-to-one correspondence between roles and geometry.

[mboeringa]

Hi @joto,

Thanks for the answer.

I have no idea how I could have overlooked it! I scrolled back and forth through the text of the manual a couple of times, but somehow missed noticing the crucial information that is there...

Now I need to figure out how to use this, but at least it is nice to know all the relevant information is accessible from Lua!

[mboeringa]

@joto,

I am afraid I still need a bit more help. I have gotten this far with the specific piece of code that should handle the selection of specific role members, or exclusion of any others.

My problem is that I do not understand if, and how, it is possible to manipulate the 'geom' variable, the 'geometrycollection', to exclude certain members, and then be able insert the entire collection (or even preferably a multilinestring') of the correct ways of role 'main_stream', into a single record in the actual database.

Yes, it is possible and I do understand how to insert individual geometry records / selected ways each in their own record, as there is an example of that using the 'insert' function in the manual.

But since I seem to be unable to remove objects from the 'geometrycollection', or create e.g. a new empty 'multilinestring' Lua geometry object and add each 'way' object I want to keep to it one-by-one, I do not see how to create and fill a single record with the entire "cleaned up" waterway relation only consisting of members of role 'main_stream'.

Of course, I could potentially insert the individual records and merge the selected ways in post-processing in PostGIS, but I was just wondering if this is possible at the osm2pgsql stage as well.

function osm2pgsql.process_relation(object)

    local type = object.tags.type
    
    if type == "waterway" then
        
        local geom = object:as_geometrycollection()
        local object_members = object.members

        for i = 1,geom:num_geometries(),1 do

            n_geom = geom:geometry_n(i)
            n_member_table = object_members[i]
            n_member_type = n_member_table.type
            n_member_ref = n_member_table.ref
            n_member_role = n_member_table.role

            if n_member_type == 'w' then
                if n_geom.geometry_type() == 'LINESTRING' or n_geom.geometry_type() == 'MULTILINESTRING' then
                    if n_member_role == 'main_stream' then
                    <NOW WHAT DO I NEED TO DO HERE?>
                    end
                end
            end

        end
        
    end

end

[joto]

No, you can't merge those single geometries in osm2pgsql again. Either import them as single entries and merge in the database, or import the information which geometries to pick into the database and then do the picking and merging in the database.

[mboeringa]

OK, thanks, clear. I can merge them at the database level after the import.

[mboeringa]

@joto

Thanks for all the help, this now succeeded. Left the new enhanced display of OSM waterway relations based on the "main_stream" members only using v1.7.0, right the previous display with the clutter of "side_stream" members in the delta of the Mackenzie River in Canada with osm2pgsql <= v1.6.0.