Polygons Parallel to Line

General description and demonstration

Depending on the settings, the plugin rotates the polygons in such a way that they become parallel to the nearest lines.

In the new layer, a “_rotated” field is created with a value 1 for each polygon that has been rotated. If in the source layer such field already exists, and its type is integer, then the data of this field remains, but updated in the new layer.

before using the plugin:

after using the plugin:

Work preparation

The provider needs to be activated in the Processing options which are accessible from QGIS main menu (Processing -> Options -> Providers). Check Activate like shown below.

Algorithm

First, the plugin creates a spatial index for the objects of the linear layer.

Within the distance specified in the “Distance from line” field, for each line of the layer selected in the “Select line layer”, the plugin finds the nearest polygon of the layer selected in the “Select polygon layer”.

If the line is the multipolyline, then each line of the multipolyline is processed as a separate.

Two edges of the polygon adjoin the node. If the angle between the edge and the nearest line segment is no larger than the value specified in the “Angle value” field, then the polygon rotates relative to the centroid so that the edge becomes parallel to the line segment.

Multipolygons are rotated by the same principle with using the nearest node and the centroid of the multipolygon.

If both the angles between the edges and the line segment are less than the “Angle value”, then for the rotation is selected the edge, which forms the smaller angle if in the settings are not chosen “Rotate by longest edge if both angles between polygon edges and line segment <= ‘Angle value’”.

If the layer objects have geometry errors, rotation may not occur.

Settings

I think Rotate only selected polygons and Do not rotate multipolygons options are not needed in explanations.

Distance from line

If value is 0, plugin will process all polygons. In the following example, the distance is 50 m (showed by grey).

before:

after:

Angle value

Value, degrees:

min: 0
max 89.9

The following example shows the angle between one edge of the polygon and the nearest line segment. The angle for the second side is similar. The plugin compares these angles with the values from the settings.

Save only selected

If not chosen “Save only selected”, to new layer will saved both rotated and unrotated polygons. This option makes sense if chosen “Rotate only selected polygons”.

Rotate by longest edge if both angles between polygon edges and line segment <= ‘Angle value’

before:

if not checked (angle near the short edge less than the long edge):

if checked: