These plots give comparisons of eccentricity between the two kinds of ellipses: one is the image seen on the pieces of CR-39 and the other is the true surface opening. The holes were etched out from alpha tracks striking the CR-39 at 0, 15, 30, and 45 degrees from normal incidence.

Eccentricity is a measure of how close an ellipse is to being a circle. 0 eccentricity represents a perfect circle, and greater values represent increasingly elongated shapes.
Eccentricity=square root[1-(major axis/minor axis)²]