# [pgrouting-users] PG Routing Question

Daniel Kastl daniel at georepublic.de
Sat Sep 2 06:58:36 PDT 2017

```Hi Andrew,

Once I had to route to every point in a virtual raster, which makes sure,
that there is a node every few hundred meters.
The first step was to create raster nodes. Then you query for the nearest
edge of each raster node.
When you know the edge you can create a link between the raster point and
the source and target of this edge and assume some costs to reach the
raster node.

Here is a picture, how it would look like:

You could also move the raster nodes onto the nearest edge.
Not sure, if this is useful for your use case, but I thought I share it.

Best regards,
Daniel

​

On Sat, Sep 2, 2017 at 10:38 PM, Stephen Woodbridge <woodbri at swoodbridge.com
> wrote:

> Hi Andrew,
>
> This is a really good question it gets asked every so often. So I will cc
> the pgRouting-users list with my response.
>
> The routing algorithm computes the cost to get to each node following the
> road network. If the cost is time based then it takes the speed of the road
> segments into account. You can't do meaningful interpolation between points
> without taking into account the edges. Think about a path following the
> letter "S" or "C",  interpolation between the end points does not take into
> account the path of the letter or in our case the roads. Also if you pick
> two arbitrary nodes they may or may not be connected directly. So
> interpolation is not going to make sense.
>
> One option might be to examine each edge, and get the values of the their
> respective nodes, then interpolate along the edge and add the interpolated
> points into a temporary table, then you can union the original points plus
> the interpolated points into pgr_pointsAsPolygon()
>
> Another solution that might work for you if you need better resolution but
> this would be the same as above and require more work, would be to take
> every edge and break it into 4 equal edges and replace the original edge so
> that instead of having one edge and 2 node, you now have 4 edges and 5
> nodes. Then you can rebuild the topology and your driving distance will
> have a high density of node.
>
> I hope this helps,
>   -Steve
>
> On 9/1/2017 10:52 AM, Andrew Wooley wrote:
>
> Stephen
>
> I attended your workshop at the FOSS4G Boston conference. I really enjoyed
> it and learned a lot. I have been working on some projects using it.
>
> I came across an issue that I was hoping that you could help me. I am
> using the following example function from Regina's workshop:
>
> SELECT
>     1 As id,
>     ST_SetSRID(
>         pgr_pointsAsPolygon(
>             \$\$
>                 SELECT D.seq AS id, ST_X(V.the_geom) AS x,
> ST_Y(V.the_geom) As y
>                 FROM
>                     pgr_drivingDistance(
>                         \$sql\$
>                             SELECT gid As id, source, target, length_m AS
> cost, length_m AS reverse_cost FROM osm_ways
>                         \$sql\$,
>                         (SELECT id FROM osm_ways_vertices_pgr N ORDER BY
> N.the_geom <-> ST_SetSRID(ST_Point(-111.69429,40.28984),4326) LIMIT 1),
>                         500,
>                         TRUE
>                     ) D INNER JOIN
>                     osm_ways_vertices_pgr V ON D.node = V.id
>             \$\$
>         ),
>     4326
>     ) As the_geom
> ;
>
> It is selecting the vertices in the table and the creating the polygon. It
> works well, but I was wondering if there was a strategy for interpolating
> points along the line to make the distance be the input distance between
> vertices even if there isn't a vertex there in the table.
>
> Thanks a lot for your help.
>
> *Andrew Wooley* | IT Director
> Mountainland Association of Governments
> 40.311,-111.682
> http://www.mountainland.org
> 801-229-3844
>
>
>
>
> www.avast.com
>
> _______________________________________________
> Pgrouting-users mailing list
> Pgrouting-users at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/pgrouting-users
>

--
Georepublic UG & Georepublic Japan
eMail: daniel.kastl at georepublic.de
Web: https://georepublic.info
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