Hello Both,

Thank you for your answers so far and I could indeed get on with the python 
methods but I’m trying to build a solution which a normal user would understand 
with minimal effort really.

I’m currently looking at using the graphical modeler to create a routing which 
will replicate (albeit inefficiently) the Travelling Sales routing by running 
shortest route in batch form. I think I got 90% of the way yesterday before I 
ran into the obvious issue with the process modelling.

Is there any way to output a string from a process which you can then use as an 
input in another process?

I ask because all the routing algorithms require at least one ‘point’ (string) 
input. I have extracted the coordinates from my points layer as a string in the 
form ‘x, y’ and I can see the string if I run that section separately but I 
can’t set the string as an input for shortest route algorithm in the modeler.

I’m thinking that the best method would probably be to run a distance matrix 
and then use that to construct a series of A – B routes in order, run each of 
those and then stick them together to create the output. Given the comment that 
“Both points (start A and end B) are “tied” to the graph when it is built”; I’m 
pretty convinced that my plan of using clipped versions of the network for the 
routing may still be the best method for QGIS.

I’m aware that this is really using a spade to dig up a road when really I 
should be just investing in a digger but the goal here is to see what can be 
achieved with QGIS 3.4 as it stands today. I’m not too fused if it takes 5 
minutes to run a 30 second job so much as whether it can be achieved accurately 
and using a method that end users might be able to understand.

Thanks,
Paul

From: Nicolas Cadieux <nicolas.cadi...@archeotec.ca>
Sent: 16 September 2019 16:09
To: DelazJ <del...@gmail.com>
Cc: Paul Wittle <p.wit...@dorsetcc.gov.uk>; qgis-user@lists.osgeo.org
Subject: Re: [Qgis-user] Shortest path caching

Hi Harrissou

I was referring to the online documentation in the developers cookbook.  The 
user guide is effective.  I perfectly understand the problem and wish I could 
help but despite receiving help from advance users with links to online code 
examples (thanks), I found it easier to go with NetworkX that was well 
documented.  Maybe I could figure it out now since I now have experience 
working with shortest path analysis and now grasp the basic concepts of 
networks but when I started the project, it was a big learning curve.

Cheers

Nicolas

Le 16 sept. 2019 à 09:36, DelazJ <del...@gmail.com<mailto:del...@gmail.com>> a 
écrit :
Hi,

Le lun. 16 sept. 2019 à 15:25, Nicolas Cadieux 
<nicolas.cadi...@archeotec.ca<mailto:nicolas.cadi...@archeotec.ca>> a écrit :
Hi,

You could go with NetworkX, a python library.  With that, you can build the 
graph on the fly and maintain it as roads or edges get added and removed.

You can probably do this with QGIS but the biggest problem I found with QGIS is 
the lack of documentation for using the network analysis tools using the latest 
versions. Everything is, or was documented for version 2.x.  Also, when using 
the processing tools in a batch, the graph gets rebuilt every time this should 
not happen.

Nicolas, about the lack of documentation, you meant the tools in the processing 
toolbox [0] or the pyQGIS cookbook [1]. I'm not sure there's anything missing 
in the former (and maybe Paul could find some answers) and, for the latter 
(which is horribly outdated), it's the usual lack of skilled people to provide 
necessary documentation. Because some parts of the docs are specialized, they 
require advanced users to help us update. If we are giving a txt (or whatever 
open format) file updating the section, I'm happy to push it and update that 
page too.

[0] 
https://docs.qgis.org/testing/en/docs/user_manual/processing_algs/qgis/networkanalysis.html
[1] 
https://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/network_analysis.html

Regards,
Harrissou
For those reasons, I went with NetworkX python library.  Out of curiosity, how 
many edges are you dealing with? How many queries?

Nicolas





Le 16 sept. 2019 à 08:03, Paul Wittle 
<paul.wit...@dorsetcouncil.gov.uk<mailto:paul.wit...@dorsetcouncil.gov.uk>> a 
écrit :
Hi,

I’m aware that you can use things like the pgRouting plugin for shortest path 
calculations but when looking at the default functions is there a sensible way 
to speed up the calculations?

I’ve just run a couple of tests using an Oracle database based routing layer 
and it runs but seems to take a long time to build the graph. I wondered if 
there is a plugin which stores some sort of optimised network to speed up the 
process or perhaps a type of index I could put on the network?

At present the network is big so I guess perhaps some form of clip then run 
process might be helpful?

I just thought I’d email to see what others do as I’ve not looked at routing in 
QGIS for some years now and I’d prefer to avoid more complex options such as 
pgRouting if possible.

Thanks,
Paul
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