# [pgrouting-users] Re: Pgrouting-users Digest, Vol 40, Issue 2

Steve Horn steve at stevehorn.cc
Mon Jan 9 20:26:27 EST 2012

```>
>
> > Hello all!
> >
> > I am creating drive time radii (or isochrones) by allowing the user to
> > click on a map surface and entering a number of minutes.
> >
> > Using the driving_distance function in pgrouting I am able to get very
> > close to a good solution, but it is somewhat limited in that you can
> > only route from node to node (vertex to vertex). In order to improve the
> > accuracy, I thought I would try to insert a new node/vertex as close as
> > possible to the coordinates that my user clicked. That way my start node
> > would always be as accurate as it can be.
> >
> > Has anyone done this? I'm not sure I understand the assign_vertex_id
> > function and how the graph is constructed well enough to come up with a
> > solution myself.
> >
> > Any help or direction is appreciated!
>
> What you would want to do in this case is:
>
> 1. take you position and find the nearest edge to that location (EID,
> ESOURCE, ETARGET, ECOST, EREVERSE_COST)
> 2. compute the ratio of distance along that edge (RATIO)
> 3. then add a create a temporary node at that location (NEWNODE)
> 4. then add edges from the temp node to each of the end points to that
> edge, paying attention to oneway-ness of that edge and assigning costs
> as a ratio of the original cost.
>
> So select edges for your query might look like and you would pass
> NEWNODE as the starting node.
>
> select gid, source, target, cost, reverse_cost from roads
> union all
> select maxgid+1, ESOURCE, NEWNODE, ECOST*RATIO, EREVERSE_COST*(1-RATIO)
> union all
> select maxgid+2, NEWNODE, ETARGET, ECOST*(1-RATIO), EREVERSE_COST*RATIO
>
> -Steve
>
>
I get lost on your 3rd step: "Create a temporary node"
How do you insert just a node? The "roads" table is made up of edges, so
wouldn't I need to insert an edge?
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