[OSM-dev] Relations without members?
brett at bretth.com
Thu Oct 11 14:30:07 BST 2007
Frederik Ramm wrote:
> The polygon excerpt script does the same. If you run it on a polygon,
> a bounding box is first computed and every node is first filtered
> against the bounding box before the polygon test is made, because when
> I designed that script I was using a 5,000+ node border polygon which
> made for slow "is-in-polygon" testing. If you run it on just a bonding
> box, then the polygon part is simply skipped and the bounding box
> defined by the user.
Fortunately for me the java Area class seems pretty smart in this
regard, I thought about wrapping it with a bounding box but it didn't
> In the most primitive mode of operation the script will not care for
> referential integrity and might output ways for which not all nodes
> are present. But you can force it (with -r) to re-read the nodes
> afterwards, adding all those that are required to make the ways
> complete. The code is engineered to allow one to easily extend that to
> as many levels as you like, i.e. you could also have it re-scan the
> ways, add any that have been used by relations, and then re-scan the
> nodes to again add any required by these ways, and so on. You can even
> operate it in a mode where it does not require ordered input but it is
> moch slower then.
That is pretty cool, not something that will be easy (or even possible)
to do with osmosis. Unless I use temp files, each task only gets one
shot at reading data. Re-reading nodes isn't simple to do.
>> Ways are selected if one or more of its nodes are in the bitset. The
>> way is modified to only include nodes that are inside the area. The
>> selected way ids are added to a bitset.
> This is something that the extract-polygon script cannot do - it
> either gives you the full way or no way. I would actually like to
> implement that functionality, plus an extra hardcore function that
> will not only drop some nodes off the way, but insert one border node
> exactly where the way intersects with the bounding box... ;-)
That would be neat :-) Again, not something osmosis will be able to do
because I only have node ids at this point, I have no idea where they
are from a coordinate perspective.
>> Relations are trickier, obviously they are selected if one or more of
>> its members are already in the node bitset or way bitset. And I can
>> modify the relations to only include nodes and ways that are inside the
> The perl script makes no attempt at modifying relations, In primitive
> mode, the perl script will simply give you the relations of which at
> least one member has been selected before; in referential integrity
> mode, it should give you the nodes and ways referenced by a relation
> as well. It does not recurse into relations, although this could be
> enabled in the code. I haven't yet found a good way for cutting off
> though - when giving out extra relations, should I also select their
> way/node type members, or will I then quickly accumulate the whole
> I would advise against removing member from relations as this might
> render them useless.
Makes sense. It's easy for me to just leave them alone, it will just
affect referential integrity ...
I'll have a crack at getting it working again, it will just have a few
"known" issues around relations. For now people will definitely be
better off with your perl script.
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