In classical geometry, the "Radial Distance" is a coordinate in polar coordinate systems (r, $\theta$). Basically the radial distance is the scalar Euclidean distance between a point and the origin of the system of coordinates.
Applicable units are those of quantitykind:Length
http://en.wikipedia.org/wiki/Radial_distance_(geometry)
$d = \sqrt{r_1^2 + r_2^2 -2r_1r_2\cos{(\theta_1 - \theta_2)}}$, where $P_1$ and $P_2$ are two points with polar coordinates $(r_1, \theta_1)$ and $(r_2, \theta_2)$, respectively, and $d$ is the distance.
http://www.iso.org/iso/catalogue_detail?csnumber=43012