Stands consisted of Douglas-fir, Engelmann spruce, lodge pole pine„ 

 juniper, aspen, and noncommercial hardwoods. Most of the conifers had 

 narrow, rather pointed crowns. The stands were well distributed through 

 a range of about 20 to 90 feet in height. Mean height of all stands was 

 about 53 feet with a standard deviation of 18 feet. The stands were 

 scattered through mountainous area with elevation differences averaging 

 about 1,500 feet on a single photo. As in many parts of the Rockies 

 precise contour maps were not available and both the average scale of 

 the photo pair, and the elevation differences of the stands measured had 

 to be estimated from the few elevations recorded on planimetric maps. 



The following rough method was used by the interpreter in estimating 

 the parallax factor :2J 



1. The elevation of the principal points of the photo pair was 

 estimated from the best available sources and the approximated mean 

 scale was then computed for the photo base. 



2. Using this scale and the length of the photo base a parallax 

 factor was read from a previously prepared table. ±£/ 



3. This factor was adjusted when the estimated difference in 

 elevation between the measured stand and the mean datum of the photo 

 base approached 500 feet. 



9/ The elevation change in feet for each 0.001-inch difference in 

 parallax measured on the photos. 



10/ This table, familiar to many foresters, gives elevation dif- 

 ference in feet for each 0.001-inch parallax difference by scale of 

 photo and length of photo base. 



-5- 



