[pgrouting-users] Question about driving_distance calculation

[Daniel Kastl]

2011/1/11 Markus Innerebner <markus.innerebner at inf.unibz.it>

> Hi all,
>
> I need some information about driving_distance calculation function:
>
> 1) the query point(source_id) has to be a vertex or can it be a 2D point
> that is mapped to the closest possible edge?
>

Driving Distance functions starts with a vertex. (There is a wrapper
function that takes coordinates as start and searches for the nearest vertex
from the given point).



> 2) what happens with edges, that are partially reached? (one vertex is
> reached in time, the other one is not)
>

Driving Distance only cares about vertices, because it works like Dijkstra
algorithm. You can take the vertices and write an SQL function for example,
that returns you the edges ... or a polygon.



> 3) is the entire network loaded into main memory for doing such a
> computation?
>

This depends on your query. You can only select the network in your query
that you think is relevant for your result. Usually you select a bounding
box around your start point. It's easy to guess how large this area is, if
you cost parameter is a distance. It's more difficult if you cost parameter
is time for example. If your bounding box was too small, then your result
might have the shape of a square.

Daniel



>
> thanks for your answer
>
> best regards
>
> Markus Innerebner
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-- 
Georepublic UG & Georepublic Japan
eMail: daniel.kastl at georepublic.de
Web: http://georepublic.de
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