Grass Processing tools via QGIS : v.clean snap, chdangle, and rmdangle

Question

Using QGIS 2.6 Grass Processing tools, I'm trying to clean up a topology on a water network layer.

I noticed topological errors when first trying to break all lines on intersections using v.clean break. Some didn't break, which makes me realize that some are not completely touching.

Which is the best to use: v.clean snap, v.clean chdangle, or v.clean rmdangle?

Lines seem to be moving for all cases tested, but I cannot tell whether the endpoints are extending towards the other feature or the feature is moving to meet the endpoint. I'm looking to do the former, if possible.

I've looked at the GRASS manual, but am still unsure:

The chdangle tool is similar to the rmdangle tool but works only on boundaries and changes dangling boundaries to lines instead of removing them.

I guess I'm unable to visualise this.

Answer 1

Sometimes it's easier to understand something with the help of graphic or images. This post explains how to clean topology error with GRASS v.clean tool with some images and explanation about the result. Unfortunately it's not explain all the process, but at least could help you to get more understanding

Answer 2

If you lines are not completely touching, this is called an "undershoot". This type of error can be corrected using v.clean snap (undershoot will be snapped to the other vertices within the tolerance.