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I'm trying to digitize buildings in QGIS and encountered a problem: after digitizing several vertices in a sketch, I want to place my final vertex in accordance with the first one - i.e 'square and finish' (in ArcMap).

Is there a way to snap\refer to a previous sketched vertices in QGIS?


The only solution I found involved creating features and then using its vertices as snapping anchors for precise sketching.

enter image description here

  • That would result in a faulty geometry, why would you want to do that? – Erik May 14 at 15:44
  • You are digitizing your building like you would with a "closed" polyline ? is there a specific reason for that that prevent you digitizing them as regular polygon ? (meaning using only 4 non overlapping vertex for 4 sided polygon) – J.R May 14 at 15:49
  • Hi @Erik, I've must explained myself unclear: the final vertex should be placed in the lower left star. Its position should be perpendicular to the lower dashed line. – EranGeo May 15 at 6:41
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There is no "snap to sketch" equivalent in QGIS, unfortunately. There are a couple of ways to deal with this situation, but each involve a couple of steps.

Go to View > Toolbars, and enable the Shape Digitizing Toolbar. One of the tools is for creating rectangles. There are several different options within that tool. For more precision, you may want to use the Add rectangle from 3 points tool.

rectangle digitizing tools

Now as you digitize shapes, they will be perfectly orthogonal. What about L-shaped buildings, though? Or more complex shapes? For that, we can combine other tools from the Advanced Digitizing Toolbar.

  1. Reshape Features reshape features icon

With your rectangle feature created, you can use its vertices, segments, and angles to reshape.

reshaping process

  1. Advanced Digitizing Panel advanced digitizing icon

In conjunction with the reshaping, you can also take advantage of the Parallel and Perpendicular modes on the Advanced Digitizing Panel. With advanced digitizing tools active, you can also press p on the keyboard to toggle through these.

Once you enable either parallel or perpendicular, your next click will select a segment to refer to, but will not place a vertex. Once clicked, your digitizing angle will be constrained. Additionally, helper lines will appear to show where your line will match up with existing lines.

perpendicular digitizing

  1. Merge Selected Features merge features icon

If you've got a simple L-shaped house, you can also digitize two overlapping rectangles, select them, and then merge them together.

  • Thank you for the detailed answer @JoshC. – EranGeo May 15 at 6:35

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