Select circular polygons in QGIS

Question

I have a multipolygon layer, in which some geometries are circles, drawn in an unknown GIS. How can I select only those features which are circular? A quick manual check confirms that circular features do not all have the same number of points/vertices.

Erik · Accepted Answer

You could use the field calculator in order to check whether the minimal circle differs much from the actual shape, e.g. by calculating the $area.
if(area(minimal_circle($geometry))<$area*1.05,1,0)
This checks, whether the the area of the minimal circle is smaller than the area of the original geometry times 1.05 (5 % tolerance accounting for different number of vertices) and will select the geometry, if this is the case.

GBG · Answer

The isoperimetric quotient of a circle in a plane equals 1.  You could explode your multipart features.  Add a new attribute and calculate the area of your polygons.  Add a new attribute and calculate the perimeter of the polygons. Then use the field calculator to generate the IQ of the polygons in the attribute table using this formula:
Q = 4 pi A / P^2

For shapefiles the polygons will never be perfect circles since each will be using points to define the shapes of the polygons but the polygons with IQs closer to 1 will be  circles.

Select circular polygons in QGIS

2 Answers

Add your own answers!

Ask a Question