For roundabout, there is a spezcial measuring surface with an appropriate radial raster.

To add a new measuring surface for roundabouts, go to “Insert/Measuring object/Roundabout“.

You make your entry with three clicks of the mouse

1. defines the centre
2. defines the inside radius of the roundabout
3. defines the outside radius of the roundabout

A radial calculation raster is used for the calculation.