"Migration Length" is the square root of the migration area.
Applicable units are those of quantitykind:Length
http://encyclopedia2.thefreedictionary.com/migration+area
"Migration Length" is the square root of the migration area.
$M = \sqrt{M^2}$, where $M^2$ is the migration area.
http://www.iso.org/iso/catalogue_detail?csnumber=31895