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First, each. of the four. data sets are subjected to a supervised 
Classification in the digital image analysis system to separate 
areas where the three principal crop tynes. of potato. grain snd 
hay are being grown. The results from each of the four years, 
are stored in separate. theme files, where each, theme pixel 
(cron type) is’identified Dy a unique binary number. A unique 
colour is associated with each of these numbers when the themes 
are displayed on the monitor. In this example the code numbers 
and the corresponding colours assigned to each crop are as 
follows: 
Potato.(P): l..-.red (R) 
Grain (Gl: 2 .r-z green. (G) 
Hay CH): 4. = hlue (R) 
Area statistics can now he generated for each Crop, if desired. 
Normally, the operation in the image analysis system is 
concluded. at this point. Fach theme file is stored on magnetic 
tape and then graphical plots produced for overlaying on hase 
mans. The actual crop rotation information is compiled by 
visual comparison of the three plots or by entering the output 
from the image analysis into a GIS for further processing. 
Implementing the information processing operation can, however, 
change all this. Various alternatives are open: 
Theme Overlay 
  
When all three theme files are displayed superimposed on the 
monitor, a mixture of the original theme colors is created 
which signals the existence or absence of erop rotation. 
A similar effect is created by displaying each theme file 
through a different colour gun of the monitor. Colour 
intensitv is modulated by the digital number of individual 
themes and the pseudo-colour composite represents the frequency 
of occurrence of the individual themes. The sequence of 
occurrence is, however, not discernable. 
Theme: overlay provides only a visual presentation of crop 
rotations Numerical processing of the theme files is needed to 
obtain permanent records. 
Arithmetic Operation 
  
In this method a digital overlay is. generated by. a 
pixel-by-pixel addition of the numerical values of themes in 
the three files. The new pixel values indicate the frequency 
Of occurrence of the three crop types during the three year 
period. Once stored on tape, a permanent graphical record is 
produced in a plotter. Three crop types over a three year 
Study period can yield a total of ten combinations as listed 
bolow: 
PPP = RR (3) GGP. = 5 (5) 
GGG. 2 8 (8) GGH = 8 (20) 
HAH = 13 (24) HHP 9 (9) 
PPpG 24 (6) HHG = 10 (12) 
PPH. -.6.(18) PGH 2 7 (19)