[Openstreetmap-dev] Node / line segment database statistics separation

[Andy Robinson]

>-----Original Message-----
>From: Tom Carden [mailto:tom at tom-carden.co.uk]
>Sent: 15 November 2005 10:52
>To: Andy Robinson
>Cc: openstreetmap-dev at vr.ucl.ac.uk
>Subject: Re: [Openstreetmap-dev] Node / line segment database statistics
>separation
>
>Andy Robinson wrote:
>> There is a growing trend that the difference between the number of nodes
>on
>> the database and the number of line segments is increasing. Currently it
>> stands at around 2% of nodes (dropped a little last night with Nick's
>> Freemap additions).
>>
>> On the basis that I would have thought there would be proportionally more
>> nodes than line segments as there are two nodes required for a line
>segment
>> and thus you gain a node at the end of each line run, plus there are
>plenty
>> of loose nodes lying around on the map. Therefore you would expect the
>> number of nodes to increase more, proportionally with time, than line
>> segments and not the other way around.
>>
>
>Not really.  We don't have line runs - we have a mesh of streets.  Your
>assumption would only be right for motorways.  It might be worth getting
>a plot of the distribution of node degrees (how many times each node
>appears in a line).  I'm pretty sure that for lots of nodes it will be
>greater than one, and I think orphan nodes will be insignificant.
>

I see your logic here. Its just that I know we have a lot of loose nodes out
there so was surprised by the difference. But if zero length segments is the
reason then that could be the cause.


>> I have also noted when editing that you can make a line segment from a
>node
>> and this seems to stick. Does it mean we have a large number of zero
>length
>> line segments on the database?
>>
>
>Do you mean it's possible to create a line with the same start and end
>node?  Yeah, we should catch that, but if nobody has done it on purpose
>then it'll be easy to delete them all at once from the database.
>

Yes, you can by accident and sometimes you don't realise that you have done
it. The node retains white in the editor when you refresh but I believe many
are invisible because they lie under proper segments. Should be easy to
check the database anyway.

>Best,
>
>Tom.
>
>


Andy