489 
1) if gopher 
tionships 
of herbaceous 
etation. Three 
fferences 
of new 
ovided the 
(mounds 
for mounds). 
Commission errors occurred when other objects in the photographed 
scene were misinterpreted as gopher mounds. The commission errors resulted 
from similar confusions contributing to the omission errors; misinterpretation 
of old gopher mounds, bare soil patches with apparent similar microrelief as 
gopher mounds, and infrequently some low-growing shrubs. 
Omission errors were high and commission errors were low for all 
interpreters, but they were consistent. In fact, there were no significant 
differences (P = .05) among interpreters for percent correct identification 
of fresh gopher mounds, percent omission errors or percent commission errors. 
Although the correlation coefficient between photo mound counts and ground 
mound counts was marginally acceptable (r values between 0.7 and 0.8), the 
ed to compare 
tointerpreted 
t. 
lack of significant differences among interpreters provided credence that the 
photo technique could be used to estimate pocket gopher populations. The high 
degree of correlation (r = 0.9) among interpreters for percent mounds counted 
as well as mounds omitted indicated the interpreters were consistently interpreting 
the photographs in the same way. 
ctives of 
dence. 
There were no significant differences (P = .05) in interpreter 
results due to apparent vegetation differences among plot locations or between 
treatments within locations. Obliterating the mounds before the photo mission 
may have caused some interpreter confusion. The obliteration of mounds resulted 
600 scale 
the 
o the soil 
entifying 
und-based 
based on 
in the interpretation of some of the disturbed areas as fresh mounds. There 
were obvious examples in the photographs which may have caused the interpreters 
to be excessively cautious. 
Based on the fact that interpreter errors were similar and there 
were negligible effects due to plot treatment and vegetation height, the actual 
photointerpreted mound counts were adjusted to ground mound counts. This was 
done for interpretation results for each interpreter on a plot-by-plot basis 
ariously 
old mounds, 
relative 
ric, although 
imes in 
d film 
m bluish-green, 
s were light 
and old 
texture, 
image. 
and the resulting ratio coefficient was applied to the Reid et al. (1966) 
equation in the following form: 
'y = .6582 / RM log (RM + 1) 
/N 
where: Y = gopher population estimate per unit area 
R = ratio coefficient: ground mound counts 
photointerpreted mound counts 
M = photointerpreted mound counts per unit area 
The mean gopher population estimates for all plots by the Reid et al. 
(1966) ground-sign count technique was 34 animals per 0.4 ha (Table 2). The 
erpreters 
ission errors 
ittle surface 
e mounds 
plants although 
on interpretation 
looked because 
ated low 
population estimate derived through photointerpretation of 1:600 scale color 
infrared aerial photographs was 33 animals per 0.4 ha or 97 percent as accurate 
as the estimate derived from the ground count technique. This relationship was 
developed after the photointerpreted data had been normalized to ground counts 
of earth mounds. 
There was considerable variation in the photointerpreted gopher estimates 
among individual plots. However, the close relationship of the collective values 
comparing the ground based estimates and photointerpreted estimates indicates 
increased sample size is accounting for among-plot variations. Therefore, the 
photointerpretation technique described is a valid procedure to estimate gopher 
populations provided ground truth is available to establish the coefficient 
between known ground mound numbers and photointerpreted mound numbers.