[GRASS-dev] vector network questions

[Michael Barton]

Hi Moritz,

I did some tests this evening after a long day over a beer (1 beer makes complex tasks easier, but more can make them more complex).

1. v.net.cent works once I realized that "between" was a reserved word. Thanks. I've changed the bug report to an enhancement request for better error messages. 

2. The lack of ability to query a network vector is a bug I think. I'm guessing it's in v.what, but am not sure. Can you test to see if you cannot query it either?

3. v.net.path works fine with your example, but I still don't know what the format should be when using cat values. Why 2 integers before the cat value? Of course, it is very difficult to ID start and stop points if you cannot query the network vector. That aside, your example is much easier to follow than the dated Spearfish one that is currently in the docs for v.net.path.

4. I'm still not sure of the proper workflow to create a network from an existing vector of lines--like a road or stream network--assuming that I'm happy to use nodes at intersections in the original vector map as network nodes.

Thanks again for your help
Michael
____________________
C. Michael Barton
Director, Center for Social Dynamics & Complexity 
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University

voice: 	480-965-6262 (SHESC), 480-727-9746 (CSDC)
fax:          480-965-7671 (SHESC),  480-727-0709 (CSDC)
www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu











On Oct 5, 2012, at 12:30 AM, Moritz Lennert <mlennert at club.worldonline.be>
 wrote:

> On 05/10/12 07:37, Michael Barton wrote:
>> Why does mouse querying of a vector network return "nothing found"?
>> There is attribute data in the associated attribute table. (GRASS 7 svn,
>> on Mac).
> 
> A vector network is just normal vector data, so if you get a "nothing found" then there is a problem.
> 
>> 
>> I thought I'd go over this in class tomorrow, but I can't follow the
>> manual. It is not clear how to transform a vector map (e.g., roadsmajor
>> of the nc_08 data set) into a network. Run v.net <http://v.net> with
>> nodes? connect? arcs? some combination?
> 
> The way the v.net.* modules are implemented, in order to be able to do network analysis with a network and specific nodes, you have to run v.net with operation connect (and a relevant threshold), connecting the nodes you are interested in with the network in a topologically correct way.
> 
> I guess it might be worth to develop some scripted fronted which allows you to just provide separate layers for network and nodes and the network analysis you want to do, and then the script does the v.net part for you. But for now you have to do this first. And from a pedagogical point of view I find it important to show the need for topologically correct data.
> 
>> 
>> v.net <http://v.net> path instructions seem to date from earliest GRASS
>> 6 days and are for linux based terminal commands, pipe and stdin. It is
>> not clear how to even enter start and end points. Or perhaps the
>> interactive points entry control is broken. I can't tell.
> 
> I have no problems entering points interactively in Debian Squeeze with 6.4.2.
> 
> Here's an example with the NC data set:
> 
> v.net input=streets_wake at PERMANENT points=schools_wake at PERMANENT output=streets_schools operation=connect thresh=100
> 
> Interactive use of v.net.path in GUI gives following command line:
> v.net.path --overwrite input=streets_schools at user1 output=paths file=/home/mlennert/GRASS/DATA/nc_spm_08/user1/.tmp/geog-pc40/30065.1
> (I entered the following into the interactive text field:
> 
> 1 1 167
> 2 3 166
> 3 656688 234205 629030 217772
> 
> The same could be put into a text file called with the file= option. Often a good idea for students so that you can debug...).
> 
> Attached a screenshot of the different GUI windows of the operation.
> 
>> I'm interested to know how this works. Perhaps I can work with someone
>> to update the docs.
> 
> At this stage, I don't really understand what is missing in the docs. Screenshots of GUI usage ?
> 
> Moritz
> <v_net_path.png>