THE ANSWERS 



To answer this question of accuracy we compared aerial photo 

 measurements with field measurements on 68 forest inventory plots in 

 southern Idaho. We determined the average height of the dominant stand 

 on each l/5-acre field plot from Abney level readings made on the plot. 

 A trained photo interpreter measured the total height of the dominant 

 stand on each acre surrounding the field plot center. He used a pocket 

 stereoscope and a parallax wedge and made measurements on photos used 

 by the field crew, without prior knowledge of the field plot data. For 

 the purposes of this study we assumed the l/5-acre field plot would be 

 representative of the 1-acre plot measured on photos. 



The Results Are Encouraging 



In the case of the common 9x9-inch 1:20,000 scale Department of 

 Agriculture photos, the standard error of estimate^/ was 10.7 feet. It 

 was 5.1 feet for U. S. Geological Survey 1:28, 0002/ scale photos, and for 

 1:31,500 scale enlargements of 1:40,000 mapping photos only 10.8 feet. 

 Correlation between photo and field heights was 0.81 for the 1:20,000 

 soale, 0.94 for the 1:28,000 scale, and 0.74 for the 1:31,500 scale. 



The following tabulations indicate the relative accuracy of photo 

 versus field stand heights: 



I. Comparison of averages of all stands measured 



Scale of Focal Number of Average height of all stands 



photos 



length 



stands Photo 



Field 



Difference 





Inches 



Feet 



Feet 



Percent 



1:20,000 



8.25 



29 57.1 



57.4 



-0.52 



1:28,000 



5.25 



15 37.5 



37.0 



/1.35 



1:31,500 



6.00 



24 55.5 



56.2 



-1.25 



II. 



Comparison of 



individual stand 



measurements 





Scale of 



Number of 



Mean deviation 



Standard error 



Coefficient 



photos 



stands 



photo vs field 



of estimate 



of 









photo vs field 



correlation 







Feet 



Feet 





1:20,000 



29 



7.8 



10.7 



0.81 



1:28,000 



15 



4.0 



5.1 



0.94 



1:31,500 



24 



7.7 



10.8 



0.74 



3/ This measure of the variation of photo stand heights about a 

 line formed by corresponding field stand heights indicates the maximum 

 error two times in three, when estimating individual stand heights by 

 this procedure. 



4/ The accuracy of parallax measurements is closely related to the 

 focal length of the camera and the flying height of the plane. In this 

 case the 1:28,000 scale photos were flown at 12,300 feet using a 5.25-inch 

 F.L. camera while the 1:20,000 photos were flown at 13,700 feet with an 

 8.25-inch camera. The difference is largely due to this. 



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