Is there a way to rotate all polygons in layer_a (green polygons) to align with the longest side of the polygon in layer_b (red polygon) without having to manually enter a rotation angle? In most cases there would be multiple polygons in both layers - the idea would be for all polygons in layer_a to align with longest side of all the polygons in layer_b

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I understand there is a similar question

But the solution is tedious for the amount of polygons I'm planning to adjust.

1 Answer 1


Using QGIS 3.28, this can be achieved with the Geometry by expression tool.


                'target poly',
        ) - @current_rotation       

The rotation angle for each feature is found by using the main_angle function on the 'target polygon' that intersects with that feature. The main_angle of the original feature is subtracted from the rotation angle to deal with varying orientations.

From the help panel of the expression editor:

function main_angle

Returns the angle of the long axis (clockwise, in degrees from North) of the oriented minimal bounding rectangle, which completely covers the geometry.

I applied this expression on the red polygon layer. The black outlines are the 'target polygon' layer.


enter image description here

Red polygons are the original geometry, blue polygons are rotated to the main axis of the polygon in 'target_poly' layer (black outline).


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