[OSM-dev] osm2pgsql bbox filter when applying diffs?

[Frederik Ramm]

Hi,

Peter Körner wrote:
>> Is there a way to process the diff files in a way that the diff only
>> contains changes in my area of interest?

> If you're fetching the diffs with "osmosis --rrii --wxc", can't you plug 
> a --bb task afzter that? Or does this task not work with 
> changeset-streams? 

The latter.

> If so, this would be the place to implement this 
> functionality, wouldn't it?

No. Imagine a relation with 500 members and someone changes a tag. The 
relation itself will be contained in the osmChange file, but none of the 
members will because they are unchanged. Any task handling the osmChange 
file will not know anything about the spatial extent of the relation.

This would only work if the geometry of ways and relations was included 
in the osmChange file, significantly increasing their size - or if the 
process which wants to cut a section out of the osmChange file has 
access to the underlying data.

With osmosis you can do a --rrii --rx --ac --bb --wx to read your xml, 
apply the change, cut out the area of interest (again) and save it. But 
that is impractical for something like minutely diff application.

It would have to be done in osm2pgsql - when applying a change, it would 
have to do the following:

* if change type = modify or delete: apply change as long as object to 
be modified or deleted is present in local data base (I think it does 
this already)

* if change type = create and object type = node: apply change only if 
node is within bbox

* if change type = create and object type != node: apply change only if 
referenced members are present in local data base

A problem arises (as always) if you have objects that are half in and 
half out; they will need special treatment. There will be difficult 
scenarios (nodes A, B created outside bounding box; way C created to 
connect A and B; all these changes are ignored; later node A is moved 
inside the bounding box in a different osmChange file; how will you now 
get hold of way C which suddenly has entered your area of interest?).

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"