PrA

PrA is a simple ad-hoc parameter to define the "non-circularity" or eccentricity of a 2D object. This quantity is simply:

${\begin{alignedat}{2}\mathrm {PRA} ={\frac {Pr}{A}}\end{alignedat}}$

Where $P$ is the object's perimeter, $A$ is its surface area, and $r$ is an effective size (radius), computed based on the corresponding circle of the same area:

${\begin{alignedat}{2}r={\sqrt {\frac {A}{\pi }}}\end{alignedat}}$

This definition of PrA is convenient, since it provides a simple measure of eccentricity. In particular, for a circle one expects:

${\begin{alignedat}{2}\mathrm {PRA} &={\frac {Pr}{A}}\\&={\frac {(2\pi r)(r)}{\pi r^{2}}}\\&=2\end{alignedat}}$

Since a circle has the minimal perimeter (for a given area), this is a limiting value of PrA:

${\begin{alignedat}{2}\mathrm {PRA} \geq 2\end{alignedat}}$

And thus any non-circular object will have a larger PrA. An infinitely eccentric object would have $\mathrm {PRA} \to \infty$.

Ellipse

If the object is an ellipse, with equation:

${\frac {x^{2}}{a^{2}}}+{\frac {y^{2}}{b^{2}}}=1$

Then the width is $2a$ and height $2b$ (we assume $a\geq b$), the foci are $(\pm c,0)$ for ${\textstyle c={\sqrt {a^{2}-b^{2}}}}$. The eccentricity is:

$e={\frac {c}{a}}={\sqrt {1-{\frac {b^{2}}{a^{2}}}}}$

The area is:

$A=\pi ab$

The perimeter is not analytic but can be approximated very roughly by:

$P\approx \pi (a+b)$

Which yields:

${\begin{alignedat}{2}\mathrm {PRA} &={\frac {Pr}{A}}\\&={\frac {P\left({\sqrt {\frac {A}{\pi }}}\right)}{A}}\\&\approx {\frac {\pi (a+b)}{\pi ab}}\left({\sqrt {\frac {\pi ab}{\pi }}}\right)\\&\approx {\frac {(a+b)}{ab}}{\sqrt {ab}}\\&\approx {\frac {(a+b)}{\sqrt {ab}}}\\\end{alignedat}}$