37,438 24,404 29,909 
0.93 0.806 0.994 0.947 
21,694 
(acres) 
  
957 Conf. Int. 
  
149,500 acres measured from U-2 photos taken in 1975. 
1/ 
of irrigated lands than other films and their color representations are 
close to those of LANDSAT color composites. On the Idaho study, we were 
fortunate to get good CIR transparencies over our test site one day 
after the LANDSAT pass. In the event that aerial photography cannot be 
obtained, the PSU boundaries can be transferred to existing maps and the 
irrigated lands be delineated completely on the ground to obtain actual 
acreage (21) within each PSU. 
If aerial photography is available, the block boundaries of the 
sample PSU's are transferred from LANDSAT to the photographs by the use 
of a transfer device, such as the Bausch and Lomb Zoom Transfer Scope. 
Then, within the PSU all identifiable irrigated lands can be drawn on an 
overlay of the air photo. Both the area of the irrigated lands and the 
total area of the PSU are carefully planimetered and the percent that 
the irrigated lands are of the total PSU area is calculated. This 
percentage is then multiplied by the area of the PSU (acres or hectares) 
to determine the actual acreage (a.). The computations are shown in 
tabular form in Table 2. For more details on the derivation of the 
probability sampling formula and the application to irrigation see 
Langley (1975) and Johnson (1977), respectively. 
Results 
The results of four independent estimates are shown on the bottom 
portion of Table 1. The actual acreage of irrigated land in the Idaho 
test site was subsequently measured very accurately by planimeter on the 
1:120,000 CIR transparencies and found to contain 149,500 acres (59,800 
ha). All four estimates of total irrigated lands were close to the 
actual irrigated acreage and well within one standard deviation of the 
estimate. Interpreter 2 had the largest variance and sample error 
(10.3%). His fifth observation (underlined on upper portion of Table 1) 
was responsible for this increase in variance; he predicted 55 percent 
irrigated from LANDSAT and the air photos measured 91.7 percent. Upon 
examination of this PSU on the LANDSAT image, we found many wheat fields 
which had lost their IR reflectance because they had been harvested 
earlier in the season. However, these were fields with permanently 
installed irrigation systems which had been irrigated earlier in the 
season. One must realize that these kinds of errors can occur. When 
they do, such errors will cause the variance and sampling error to be 
inflated. Such errors can be minimized if the lower level aerial 
photography or ground examination is made close to the time of the 
LANDSAT overpass. 
All computations can be made on a small desk or hand computer. The 
method is surprisingly accurate and can be repeated with equally good 
results. The photo interpretation, measurement of areas and computations 
takes 12 to 15 hours to complete. If countries with developing irrigated 
agriculture would be satisfied with sampling errors of 6 to 10 percent, 
this technique should be considered.