How to extract the polygon of an administrative boundary?
If your admin_level relations contain admin_level+x relations as subareas (a very good idea), you can download whole chunks of boundaries going from one to the parent or child. You may save your work to an osm.file. But if you reload that file later, do: to keep your data in sync, else you may get nasty conflicts to solve.
How can I extract coordinates from overpass API?
Overpass API allows you to extract by polygon: which should fill your needs. You can use QGIS to draw the desired polygon, then extract the coordinates with the MMQGIS plugin. Finally I decided to use Quick OSM plugin, which allows you to make a query based on a territory name or a layer (which can contain vectors).
Is it good to simplify a polygon before extracting it?
If you intend to use the polygon for making extracts, it may be wise to simplify it before use as extracting may take very long if the polygon is too big. Simplifying the polygon might, however, clip off bits, and you might want to look for a simplification algorithm that guarantees that the simplified polygon contains the original polygon.
Why are both ways and nodes untagged in OSM?
Both ways and nodes may be untagged if they are members of a relation. Within the OSM database, we store these attributes for nodes, ways and relations. Your application may not need to make use of all of them, and some third-party extracts produced from OSM data may not reproduce them all.
How to extract a region from an OSM file?
Create geographical extracts from an OSM data file or an OSM history file. The region (geographical extent) can be given as a bounding box or as a (multi)polygon. There are three ways of calling this command: Specify a config file with the –config/-c option. It can define any number of regions you want to cut out.
Which is bounding box do you use in osmium?
Osmium must check each and every node in the input data and find out in which bounding boxes or (multi)polygons this node is. This is very cheap for bounding boxes, but more expensive for (multi)polygons. And it becomes more expensive the more vertices the (multi)polyon has. Use bounding boxes or simplified polygons where possible.