The yellow areas represent the field average, the blue colour represents that part of the field with below average yields and the red colour is the portion of the field with above average yields. Visual inspection of the maps indicates that below average yields seem to occur in roughly the same parts of the field each year. To begin the process of making the management unit map the yield maps need to be normalized. To do this you need to go to your yield data file and calculate the field average. Divide each yield value by the average and multiply by 100 and then recreate your maps using the normalized yield rather than the yield.
Blue areas have below average yields.
These maps look very similar to the maps in Step 1 . At this point the 3 normalized yield maps are averaged. Figure 7 is the yield spatial trend map which shows over the three years with three different crops where yields tend to be below or above average. Things get a little complicated here. Unfortunately I haven't got very much experience with the mapping programs that are being used out there and thus I don't know how easy this would be to do using another mapping program.