On LANDSAT color composites which are formed by transmitting blue, 
green and red light through LANDSAT wavebands 4, 5 and 7 respectively, 
irrigated fields appear brilliant red because of the high infrared 
reflectance of the vegetation. These irrigated fields can be easily 
separated from non-irrigated lands because of the regular geometric 
shapes and red hue. A suggested procedure follows: 
Equipment 
A binocular microscope, set at a magnification of 10 X to 15 X, is 
the best instrument to use in assessing each block for the amount of 
irrigated land. A 10 X hand lens may be substituted if a microscope is 
not available. 
Interpretation of LANDSAT transparencies 
Examine each block (PSU) in the grid in a systematic fashion by 
rows and columns and estimate the amount of irrigated lands present to 
the nearest five percent. When the irrigated land estimate falls below 
5 and above 95 percent, estimate to the nearest 1 percent. Record your 
estimate on a form similar to that shown in Figure 3. If estimates of 
other land uses are desired--such as rangeland, forestland, etc., their 
percentage occupancy should be recorded, keeping in mind that the PSU 
total should be 100 percent. As each block is interpreted, add the 
estimated percent of irrigated land to that of the preceding block 
(column four of Fig. 3). In other words, accumulate the percent of 
irrigated land as the interpretation proceeds. 
Selection of Sample Blocks (PSU's) 
Next, one must make a selection of some predetermined number of 
PSU's from the total number of PSU's examined. On the Idaho test site, 
we had 9 samples from a total of 270 which represented 3.2 percent of 
the area. Often one is restricted by lack of time and money from 
conducting an adequate ground check. Usually, a five percent sample is 
adequate. 
In making the selection of PSU's, the investigator picks the 
sample from a random number table. The numbers will run from zero to 
the total accumulated percentage of irrigated lands (final number in 
column 4, Figure 3). On the Idaho test with 270 PSU's, this total 
percentage exceeded 5,000. The random numbers will determine which row 
and column (PSU) is selected. All PSU's with one or more percent 
irrigated land have an opportunity of being selected; see the wide range 
in predicted percentages of irrigated lands for interpreters three and 
four (Table 1). 
Supplemental Aerial Photography 
One must determine the actual area of irrigated lands (a.) within 
the selected PSU's to compare it with the estimated amount from LANDSAT 
interpretation. Any scale of aerial photography (1:125,000 or larger) 
may be used to provide this comparison. Photos should be taken over the 
randomly selected PSU's during the same time period if at all possible. 
CIR transparencies are preferred because they provide better discrimination 
   
    
  
  
   
  
   
  
  
  
  
   
  
  
  
  
  
  
  
  
  
  
  
   
  
  
   
  
   
  
   
  
  
  
  
  
  
  
  
   
  
  
  
  
  
  
    
LANDSA 
TEST 4 
Colum