[GRASS-user] GRASS Connected Components

[Daniel_Victoria]

Hi Talha,

It’s been a while since I’ve done that analysis. So my memories are not all 100% certain.

Reading the email tread and some annotations I have, the main problem was that there were undershoots and overshoots in my network (as mentioned by Mark). Since my road network wat not that big, I did some manual corrections in the network.

My process was something along these lines (I could have missed a step in my notations):

1. Import the vector road network with appropriate snapping distance (my was 0.001 - lat lon location)

2. Do topology cleanup (break, rmdup and snap)

3. Create a network (v.net) putting the road network in layer 1 and my end points in layer 2

4. Run v.net.iso to find the equal distances to each end point and paint the result using cat values as the color attribute. Lines that can’t reach an end point will have no attribute

5. Inspect the lines that can’t reach an output and correct the connections (use grass digitizer and look for undershoots and overshoots. Break lines and trim/extend as needed)

6. Repeat steps 3 - 5 until happy

7. …

8. profit

Of course, if your road network is very large, this approach might not be the best option since I recall spending a couple of days manually fixing (and cursing) the road network

But in the end the work was worth it. I was able to calculate the average distance of several soy producing areas to Biodiesel crushing plants in MS, Brasil and that was used in a paper about Land Use change and Life Cycle assessment.
http://dx.doi.org/10.1007%2Fs10098-016-1161-8

I’m replying also to the grass list, in case someone has a better alternative

Cheers

Daniel

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On Tue, Mar 14, 2017 at 2:46 PM, Talha OZ <sadece@gmail.com> wrote:

Hello,

Sorry for spamming you, I saw your thread here re fixing road topology.
I managed to v.clean.advanced (break, rmdupl, rmdangle, snap) but when I run v.components I see some apparently connected lines get different values of component.
This might be due to the cat issue you talked about but I couldn’t get that part of your conversation. Could you please elaborate a little on that or direct me to a documentation, please.

I searched for v.cat… on toolbox but couldn’t find it. Is v.category option=add to be called during cleaning or finding the components?

Really appreciate your help. Thank you!
When this is the settings:

For example, these two lines get two different comp values:

Note: interestingly, when I try to save the results directly to the shapefiles I got this weird result (I think it somehow finds the original geometries and runs components on it)

-Talha Oz