Restricting the creation of lines for network creation in QGIS: start/end vertices must snap to existing vertices

Question

My interest lies in network creation, however, knowing how to do that in general would be a plus. How is it possible to prevent a user from creating a line (line vector) without connecting it perfectly to nodes (point vector) on both ends?

This would save tremendous time in topology checks and corrections later. I know about using snapping, but it is sadly not fool proof.

Answer 1

The idea: use virtual fields

To restrict the creation of lines to lines with start- and end points snapped to existing vertices, create a virtual field (updated whenever you add a feature) that checks if start- and endpoint are on a line segment. Then you have two options:

1. Set "hard" restraints - adding an unconnected line is not possible. Only lines connected at start- and endpoint will be added.

2. No restraints for adding lines, but unconnected lines will be highlighted visually until you connect them - this gives you a visual control where there are unconnected lines left and allows for a more intuitive workflow.

Virtual field: the principles

For both options, first create a virutal field (then it is updated every time you add a feature). This field tests if the start-point is connected to a line. Name the field as start, set field type to boolean. Repeat the same for the end-point, creating a virtual field end.

Use the following expression for the startpoint (at the beginning of line two replace start_point with end_point to create field end):

intersects( 
    start_point ($geometry),
    difference (collect ($geometry), $geometry)
)

Option 1: Setting hard restraints using constraints

Go to Layer Properties / Attributes Form and select the virtual fields. For Constraints, set (again) the expression from above for start- and end field. Be sure to check the box Enforce expression constraint.

Now, you won't be able any more to add unconnected lines. Only fully connected lines will be accepted.

Setting the constraints:

Unconnected ends: you can't add the feature, OK button is grayed out:

Option 2: Visual control of unconnected features

Style the lines differently based on the result of that field. Like this, you have a visual control where there are unconnected lines left and you can connect them.

Set the layer's style to Rule-based and add two rules, one for "start" = true and "end" = true (both points connected to the network: green), the other for "start" = false or "end" = false (one or both points not connected to the network: red).

Now, you can visually recognize which lines are connected and which ones are not. Connecting unconnected lines changes their visual style as you can see in this screencast:

Red: unconnected lines; green: connected lines. If unconnected lines are connected, their style changes. That gives a visual control of where there are unconnected lines and helps you to intuitively complete the lines:

---