osm2pgsql-expire: Expire type=boundary relations as multipolygons

You can always use --mode=boundary_only to switch back to boundary only
interepretation.

So type=boundary is now treated exactly like type=multipolygon.

Also adds more information to man page.

Author: joto

man/osm2pgsql-expire.md

@@ -16,7 +16,8 @@ The expire command can be used for two things:
 1. **To check what tiles some OSM data is in.** If an *OSM-FILE* is specified
    osm2pgsql-expire will calculate the tiles covering the objects in that file.
    Note that the file must not be a change file but a regular OSM data file!
-   Output is, by default, a tile file, but GeoJSON is also possible.
+   Output is, by default, a tile file, but GeoJSON is also possible.Covering
+   tiles are only calculated for tagged objects.
 2. **Visualize tile list.** If a *TILE-FILE* (presumably generated by osm2pgsql)
    is specified, a GeoJSON file is generated showing all mentioned tiles. In
    this mode all command line options are ignored.
@@ -25,6 +26,25 @@ Read the *Expire* chapter of the osm2pgsql manual
 (https://osm2pgsql.org/doc/manual.html#expire) for details on how to
 interpret the `-m, \--mode` and `\--full-area-limit` options.
 
+# GEOMETRY GENERATION
+
+To calculate the tiles covering the specified OSM data in mode (1), a geometry
+has to be generated first for each object. This works as follow:
+
+* For nodes a point geometry will be generated.
+* For closed ways a polygon geometry will be generated, for non-closed ways
+  a linestring geometry. If a generated polygon is not valid, the program
+  will fall back to a linestring. If neither a valid polygon nor a valid
+  linestring can be generated the object is ignored.
+* For relations a geometry is generated based on the `type` tag. For types
+  `multipolygon` and `boundary`, a multipolygon geometry is created, for types
+  `route` and `multilinestring` a multilinestring geometry is created, for
+  all other types a geometry collection is created.
+
+Note that for (multi)polygons it depends on the command line options \--mode
+and \--full-area-limit whether the tiles calculation is based on the area or
+the boundary of the (multi)polygons.
+
 # OPTIONS
 
 This program follows the usual GNU command line syntax, with long options

src/osm2pgsql-expire.cpp

@@ -189,7 +189,7 @@ void output_expire_t::relation_add(osmium::Relation const &relation)
     osmium::memory::Buffer tmp_buffer{1024,
                                       osmium::memory::Buffer::auto_grow::yes};
     geom::geometry_t geom;
-    if (type == "multipolygon") {
+    if (type == "multipolygon" || type == "boundary") {
         log_debug("Creating multipolygon from relation {}...", relation.id());
         geom::create_multipolygon(&geom, relation, buffer, &tmp_buffer);
     } else if (type == "route" || type == "multilinestring") {