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In QGIS, I'm trying to draw some polygons (mostly rectangles) on a grid, with neat 90-degree right angles and such. All the polygons should be parallel or perpendicular to each other.

The problem is that the city I'm working on top of, Chicago, skews a little bit off a proper north-south line. For example, in the image below, Grant Park's field (yellow rectangle) is off just a bit from the red north-south line generated by Advanced Digitizing (and the red rectangle built from that line). Thus, anything I create off that red line won't match up with the park's grid.

Can I somehow "rotate" or "offset" the tool such that every line snaps to the yellow rectangle's lines, instead of compass north? If I add a bunch more red rectangles, their lines should all be parallel and perpendicular with the yellow one over Grant Park.

I tried rotating the entire QGIS canvas, but Advanced Digitizing still just points north-south. Or, if not through Advanced Digitizing, can I create my own guide/grid lines to snap other polygons to? Not really sure the proper way to do this.

illustration

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You can specify the angle of the first edge of the feature you're digitizing in Advanced Digitizing mode by digitizing the first point of your feature, pressing a, entering the desired angle, and pressing enter. After that, if angle snapping is activated and the "Relative angle" mode is active (use Shift + a to toggle if necessary), all edges will be drawn parallel or perpendicular to this first edge:

Unfortunately, the specification of the starting angle has to be repeated for each new feature - it would be nice if it were possible to specify a fixed angle offset. Consider opening a feature request for this.

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  • Thank you! (And sorry for the slow response back). That was a very helpful explanation and animation. I didn't realize all those icons and fields were actually UI elements (relative angle, inputting a specific angle, etc.)... thought they were just decorative elements. One follow-up, if i may please: Is there a way to make the 4th corner perpendicular to BOTH the 1st and 3rd (e.g. 90 degrees to both)? I know I could've started with a rectangle shape, but that's not always possible for more complex polygons.
    – i-know-nothing
    Oct 19, 2020 at 16:51
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    @i-know-nothing: I'm glad my answer was helpful. To ensure that the fourth corner is at a right angle to the first and third, you can use the self-snapping feature that's been introduced in QGIS 3.14. I use that approach in my animation: activate self-snapping (the button on the far right of the screen), draw the first three corners, then snap the angle of the third segment to 90° relative to the second segment, lock that angle, and (using Alt+a) and then snap to the first vertex. You can see another animation at github.com/qgis/QGIS/pull/36200
    – Jake
    Oct 19, 2020 at 17:39
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    Got it. Thank you again!
    – i-know-nothing
    Oct 20, 2020 at 15:18

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