The Spin Index is the average Euclidean distance squared from all interior points to the centroid of the shape. The Spin index is normalized using the Equal Area Circle (EAC) as follows...

The Spin Index is similar to the Proximity Index but gives points farther from the center a greater weight. The results of the unnormalized index are not very intuitive. The Spin Index is appropriate to use when...

- greater sensitivity to the polygon's outlying area is needed
- the distribution of the shape about the center is relevant
- a measure of compactness is needed (normalized Spin Index)

Spin Index results for select shapes...