[Tagging] Relation for saying "x is attached to y"?

[Sebastian Klein]

Tom Chance wrote:
> Hi there,
> 
> I want a way of creating an object for an energy generator and then 
> saying "this is on top of / inside this building object". Here is an 
> example of one such object:
> 
> http://www.openstreetmap.org/browse/way/35802300
> 
> I checked on the wiki page and couldn't see any established or proposed 
> uses of relations for this purpose. Am I missing one, has this been 
> discussed before?
> 
> http://wiki.openstreetmap.org/wiki/Relations

Isn't it kind of obvious, that a "photovoltaic" type power generator is 
located on top of the building rather than inside or below?

You may assume a basic level of intelligence from the user of the data 
and add only information that has more than one option. E.g. is the post 
box attached to a wall or does it stand on its own? In this example you 
would not need a relation, since the post box node can simply be joined 
with the building way.

If there is something notable about the object, you could also start 
with a note=* tag that describes the interesting features. If there are 
more advanced tagging schemes in the future, other mappers can apply it 
without visiting the place again.

Sebastian