mmMomnifi forest & uhe expemem station 



OCDEN UTAH 



No. 25 



December 1955 



THE ACCURACY OF STAND HEIGHT MEASUREMENTS 

 ON AERIAL PHOTOS IN THE ROCKY MOUNTAINS 



Karl E. Moessner 

 Division of Forest Economics 



THE QUESTIONS 



Stand height is one of the most useful measurements foresters can 

 make on aerial photos. Height correlates well with volume, and thus 

 becomes important to the aerial estimator. Heights can be measured on 

 photos much faster than on the ground. The pocket stereoscope and paral- 

 lax wedge are far cheaper to use than the Abney level, but what of their 

 comparative accuracy in rough country? 



Most studies comparing photo with field height measurements con- 

 sider individual trees and have been made in areas east of the Great 

 Plains or in Canada. Little has been written about the accuracy to be 

 expected from photo measurements of stand height of coniferous stands 

 of the Rocky Mountains. Spurri/ has pointed out that the height of a 

 conifer is more difficult to measure than the height of a hardwood 

 since the tips of tapered crowns characteristic of conifers are often 

 too small to regist er on the 1; 20, 000 scale photos in common use. 

 Rogers^/ has shown that tree height measurements by parallax may have 

 errors up to 25 feet in areas of high relief, if no corrections are 

 made for elevation differences. However, the tops of clumps or stands 

 of conifers have a density not presented by individual trees and in 

 most cases do resolve. Also, it is possible to make adjustments for 

 differences in elevation, even where these differences must be esti- 

 mated. The question is: Can the heights of conifer stands in the 

 Rocky Mountains be measured by parallax on available aerial photos 

 with sufficient aocuracy to justify the use of this procedure? 



1/ Spurr, Stephen H. Aerial photographs in forestry, p. 237. 

 Ronald Press Company, New York. 1948. 



2/ Rogers, Earl J. Use of parallax wedge in measuring tree 

 heights on vertical photos. Research Note No. 1, Northeastern Forest 

 Experiment Station. 1946. 



Problems of Measuring Heights 



