How significant are these differences? Because of the way in which 

 the data were collected this aspect of the problem was examined only on 

 the 24 locations where 1:31,500 scale photos were used. On 15 of the 24 

 there was no significant difference at the 5-percent level between mean 

 stand height measured on photos and that obtained from tree heights re- 

 corded by the field crew. Most stands have a considerable variation in 

 the height of the dominant trees. Other studies in the Rocky Mountains 

 indicate mean standard deviations of field heights from l/5-acre plots 

 in excess of 6 feet. The standard deviations on the 24 plots considered 

 averaged 7 feet, compared to the standard error of estimate from photos 

 of about 10 feet. Photos flown to specifications which would reduce the 

 standard error of estimate below the standard deviation of the stands 

 should allow us to determine mean stand heights from photos almost as 

 preoisely as from field plots. 



A note£/ published by Central States Forest Experiment Station 

 records a similar test. Part of the basic data for that note was a 

 series of 38 stand height measurements made by the same photo inter- 

 preter on 9x9-inch 1:20,000 scale photos of southern Indiana. The 

 photo measurements were not significantly better than those obtained 

 in the Rocky Mountain test and the standard error of estimate was about 

 the same. 



There Are Problems 



The accuracy of stand height measurements on aerial photos is re- 

 lated to the ability of the interpreter to match the floating line of 

 his wedge with the ground level and with the point he considers to be 

 the average height of the dominant trees. Most interpreters find it 

 is easy to determine the level of dense flat-topped crowns common to 

 hardwoods, but much more difficult to see and measure the ground line. 

 The same effects occur in dense evenaged stands of conifers where the 

 tips resolve but few holes can be found through which the interpreter 

 can match ground level. On the other hand, the tips of the crowns in 

 open conifer stands are hard to define but the ground level is usually 

 quite obvious. One condition tends to compensate the other and there- 

 fore stand height measurements may be about as precise in the conifers 

 of the Rockies as in the hardwoods of the East provided rough correc- 

 tions are made for estimated differences in ground elevations. 



5/ Moessner, Karl E., F. Dean Brunson, and Chester E. Jensen. 

 The accuracy of stand height measurements on air photos. Station 

 Note No. 59, Central States Forest Experiment Station. March 1950. 



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