Deleting part of a closed polyline

[Edwin Humphries]

Hi all,

This is probably a very dumb question, but I have a closed polyline that should not be closed. It forms a segment of a circle, but the feature should be just the arc of the cicle, without the chord.
How do I delete just the chord?

Thanks in advance.

[Mark Arnup]

You could use a trinity tool from the QAD plug-in, QAD is QGIS's CAD tools.

Mark Arnup
35 Havelock St
Beaufort 3373
Mob: 0417 510 853

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[Andrew Keith]

Hi Edwin,

There’s no dumb questions. The solution depends on the layer’s geometry type.

If unsure, Open the Layer Properties:

1. Double-click the layer in the Layers Panel to open the “Layer Properties” window.
2. Check Geometry Type: In the Information tab, look under "Information from Provider" for the “Geometry” type.

For a Line/Polyline Layer:

you can split the chord and adjust the vertices. Follow these steps:

1. Select the segment you want to edit.
2. Click the Split Features tool on the Advanced Digitizing toolbar.
3. Right-click on one side of the chord, then right-click on the other side, and left-click to complete the split.
4. Use the Vertex Tool to delete both sides of the chord.

If the arc is split due to the operation, you can merge the arc segments back together:

1. Select both arc segments.
2. Click the Merge Features button on the Advanced Digitizing toolbar.
3. Click OK in the Merge Features dialog.

For a Polygon Layer

Since polygon layers only support closed shapes, you cannot remove individual segments. Instead, consider one of these options:

Option 1: Add a Line Layer

1. Add a separate Line Layer to your project.
2. Trace over the arc of the circle that you need (ref step 5 below).

Option 2: Use an offset to create a guide for drawing an arc shaped polygon

If you don’t want to add a new layer, you can use an offset to create another segment as a guide then draw an arc-shaped polygon:

1. Duplicate the feature: Select the feature, then press CTRL+C and CTRL+V to create a stacked duplicate.
2. Click the Offset button on the Advanced Digitizing toolbar.
3. Right-click the segment, then drag the offset inside or outside. Right-click again to confirm.
• If desired, you can set the offset distance manually in the pop-up dialog by typing a value and pressing Enter.
4. You should now see two overlapping arcs (if not, set your fill style to No Brush).
5. Trace the arcs to create a polygon:
   a. Click Add Polygon.
   b. Enable the Tracing Tool from the Snapping toolbar.
   c. Left-click anywhere on one of the segments (eg the inside arc), then trace around both arcs to form the arc-shaped polygon. The tracing tool will automatically add vertices on top of the features you trace. I found that I needed to click every so often around the arc as I traced it. When done, Right-click to end
   d. Delete the redundant segments.

Option 3: (not recommended)  you could simply use the vertex tool to drag the middle vertex of the chord out to snap to a vertex on the arc, and continue around each vertex, but that is both tedious and will result in invalid geometry. It should look ok visually but is not advisable if you will be performing any analysis on the layer (you may get Invalid Geometry errors).

Thanks

Andrew.

 

 

Andrew Keith
Asset Management Coordinator
p 02 6969 4836 | m 0423 745 618
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[Reuben Wells]

Snapping tool and the split lines function works too. 

The split tool is a bit buggy though - you need to save your changes after each split otherwise QGIS will crash. 

Once done just select the bit you don’t want and delete it. 

Ag Logic Pty Ltd

0448 947 286

19 Marlborough St, Longford TAS 7301

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From: australian-qg...@googlegroups.com <australian-qg...@googlegroups.com> on behalf of Mark Arnup <mark....@gmail.com>
Sent: Friday, March 21, 2025 12:59:47 PM
To: australian-qg...@googlegroups.com <australian-qg...@googlegroups.com>
Subject: Re: [QGIS-Australia-User-Group] Deleting part of a closed polyline

 

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[Edwin Humphries]

Thanks for the help Andrew