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I just ran into unexpected behavior while editing a polygon shapefile in QGIS 2.8.2.

Scenario:
A polygon:
enter image description here

Then I cut that polygon into two seperate poygons: enter image description here

Then I use the Node Tool to move a vertex: enter image description here

And that's my question. Why would this create a gap? This means that the Cut tool creates two edges, which is definitely not how it should be, as edges should always be shared, hence reducing the complexity of the dataset.

I have tried to select both vertices, but that does not work; only the one on top can be selected.

Is my only option to activate snapping and move both vertices individually or is there a tool or plugin that can do this?

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2 Answers 2

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Figured it out, and it had to do with my understanding of the shapefile format. I was not aware that each feature has their own seperate boundaries. I used to think that one edge can be shared by two adjacent polygons.

To have both lines/vertices move at the same time all you have to do is enable topological editing in the snapping options.

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    As you suspect, has to do with how data is stored. Shapefiles and geodatabases use individual, independent geometries. The older Esri Coverage format worked like you were thinking, as does GRASS I believe - nodes are stored and geometries are created from stored node lists. As you discovered, separate geometries requires topological editing (QGIS or ArcGIS). As someone who never really worked with coverages, I would have the exact opposite reaction as you (ie, expect two separate lines to be created and be surprised if I couldn't 'pull the shapes apart').
    – Chris W
    Commented Jul 3, 2015 at 0:33
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    Related: gis.stackexchange.com/questions/113894
    – Chris W
    Commented Jul 3, 2015 at 0:36
  • Thank you for the clarification and for the reference; that link is actually very interesting and helpful. I learned in some class that GIS data formats are so smart because they share edges, and it really took me this long to learn that this is not a universal truth, and very dependent on the format. It's been a very eye-opening week, and I think I need to shoot my teacher an email ;-)
    – BritishSteel
    Commented Jul 3, 2015 at 7:37
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I don't think this is a bug as currently, you're selecting to move the vertex of the left polygon which is similar to reshaping it:

Left polygon

With the Node Tool, first select a node from the right polygon so that the Node tool knows which vertices belong to it. Then move the same vertex as before:

Right polygon

There shouldn't be a gap anymore, instead you would be overlapping the left polygon with the right polygon.

Hope this is somewhat clear... :)

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    @BritishSteel - Actually let me know if I have misunderstood your question and I will remove this post as I am now guessing you were looking to move the vertices so that both polygons would be affected?
    – Joseph
    Commented Jul 1, 2015 at 10:23
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    Problem solved, see my answer. It had to do with my understanding of how shapefiles work internally. I was surprised that a tool like cut actually adds two coincident lines (one edge for each feature), instead of adding one line that is shared by both features. Using topological editing solves the issue; I had overlooked that option ;-)
    – BritishSteel
    Commented Jul 1, 2015 at 11:07
  • @BritishSteel - Awesome, glad you found the solution! Yes, I too thought that the line would be shared (would make life a lot easier!), I'll have to keep this in mind whenever I cut features :)
    – Joseph
    Commented Jul 1, 2015 at 11:10

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