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

[Tom Carden]

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 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.

Best,

Tom.