Although there were no significant differences (P = .05) between film 
types, the interpreters preferred using the color infrared film. They were 
able to discriminate between vegetation and other features in the scene 
more readily with this film as compared to using the normal color film. 
CONCLUSIONS 
Photointerpretation of 1:600 scale color infrared aerial photographs 
for fresh northern pocket gopher earth mounds provided estimates of gopher 
populations 97 percent as accurate as estimates derived from ground inventory. 
The ground inventory technique employing ground sign relies on the relationship 
between number of earth mounds and earth plugs made by pocket gophers and the number 
of pocket gophers trapped on a given area. Earth plugs could not be interpreted 
in the photographs. The mathematics of the ground inventory technique were 
slightly modified for the photo inventory technique to include a ratio coefficient 
of ground sign to photointerpreted sign. 
Correlation coefficients between the number of mounds counted on 
the ground and those counted by three interpreters on the photos were marginally 
acceptable (r values between 0.7 and 0.8). However, omission and commission 
errors were consistent among interpreters; there were no significant (P = .05) 
interpreter differences. This indicated that the interpreters were essentially 
reading the photographs similarly. Therefore the ratio coefficient to adjust 
photo counts to ground counts was appropriate to establish estimates of northern 
pocket gopher populations by using the sign-count (earth mound) technique. The 
equation used was: 
Y = .6582 Arm log (RM + 1) 
where: Y = gopher population estimate per unit area 
R = ratio coefficient: ground mound counts 
photointerpreted mound counts 
M = photointerpreted mound counts per unit area 
Consequently, for potential operational procedures, some ground truth will 
be required to obtain the ratio coefficient (R) and to determine sampling 
requirements for assessing and monitoring northern pocket gopher populations. 
ACKNOWLEDGEMENTS 
The authors extend sincere appreciation to Vincent H. Reid, Research 
Biologist, U.S. Fish and Wildlife Service, for invaluable technical assistance 
and to the Forest Remote Sensing Project, Pacific Southwest Forest and Range 
Experiment Station, USDA Forest Service for exposing and processing the aerial 
film.