best year for us to identify zones was 2012, up till that year we were identifying zones by soil types, which are from maps that were drawn in the 50's, close but sometimes misidentified and can be off by a quite aways.  the yield maps from 2012 helped to get these soil type zones in the right spot, now we just got  to get the production potential associations right.        one of the posters mentioned ndvi, and biomass assessments by satellite. they are tools, but they have to be used with great care in the use of making management zones.      we too hand draw most of our management zones.    one project I was going to tackle, if the fall allowed, was to rig up one of our utvs with the capability to map via gps, and go out a physically drive the various management zones in several fields, to try to improve the accuracy of our zone maps.   also in our neck of the woods the elevation maps work for some fields, but not all.