built in strategies for getting Way/Relation locations #6
Description
In the OSM data structures, only Nodes carry a location. Ways and Relations reference a set of nodes - so their location exists implicitly through the location of their nodes.
This is arguably a usefully compact and DRY usage of data structures, but when you're actually trying to do something useful with OSM data, you'll need to get the geometry for each Way / Relation.
Currently osmio mostly leaves this up to the client code to implement, but I think it'd make sense to provide osmio users with a blessed way to do this.
The algorithm tends to look something like this:
- First, store each Node's location, indexed by the Node's
node_id - Then, for each Way, take it's
node_ids, and use them to look up the stored locations, copying them onto the Way.
This is conceptually simple, but becomes non-trivial when dealing with a range of file sizes. For very small files you probably want to just store everything in some kind of hashtable in RAM, but for very large files (e.g a continent.pbf or planet.pbf) this could require many GB. If you can afford it, you still might want it all in RAM, but for resource constrained environments, there should be a way to output the node locations to file. (nodestore seems to exist to this end).
I don't know exactly what the interface would look like, but I'd propose adding a couple of different strategies and documentation for hydrating Way and Relation locations into osmio.