It is important to have proper calipers with a large flat surface or other device because of the valleys between rope strands.  The diameter must be made at consistent places along the rope lengths (ie. 100ft, 300ft, 500ft, etc).  Note: take special care at commonly problematic areas (ie. areas where rope is tangent to sheave or hoist when in loading positions).  Secondly, when taking the measurements take them at 90 degrees to each other and then average measurements at each position.  Contact us at info@northernstrands.com or call 306-242-7073 for more information.