Complex Equipment Is Not Needed 



Foresters often wonder whether the accuracy of height measurements 

 might not be considerably improved by the use of more complex equipment 

 instead of the pocket stereoscope and parallax wedge used in this test. 

 Spurr£/ quotes a series of height measurements made in Germany using a 

 Zeiss Stereoplanigraph and other precise photogrammetric means on large- 

 scale (1:4,000-5,000) photos. These tests resulted in a standard error 

 of estimate of about 2 percent, a high degree of accuracy. Comparable 

 precisenessZ/ on the 1:20,000 scale photos used in the Rocky Mountain 

 test would result in a standard error of estimate of about 14 percent. 

 The error obtained by pocket stereoscope and parallax wedge was 16 per- 

 cent. The more complex photogrammetric equipment is chiefly designed 

 for mapping purposes, where its advantages far outweigh its cost. But 

 in the rather simple measurement of average stand heights this exper- 

 iment as well as many others seems to indicate little advantage over 

 the parallax wedge in the hands of a trained interpreter. 



Summary 



This study indioates that: 1. Stand heights can be estimated almost 

 as well from parallax measurements made on aerial photos, as from a few 

 Abney level readings made in the field. 2. Average stand heights can be 

 measured on aerial photos in this region about as precisely as in the more 

 level areas of the East if correction is made for elevation, and there- 

 fore should be equally usable. 



PROCEDURE FOLLOWED 



The photos used were of three different scales, flown with cameras 

 of three different focal lengths: 



Scale: 1:20,000 (Approx.) F.L. 8.25 inches 

 Scale: 1:28,000 (Approx.) F.L. 5.25 inches 

 Scale: 1:31,500 (Approx. )£/ F.L. 6.00 inches 



All photos were 9x9 inches. 



6/ Spurr, Stephen H. Op.cit., p. 241. 



1 1 Accuracy is largely dependent upon the parallax and scale of 

 photos and focal length of the camera used. Using a 6-inch mapping 

 camera, 1:4,000 scale photos would have a factor of about 0.6-foot 

 elevation change per 0.001-inch parallax difference. Using an 8.25- 

 inch camera 1:20,000 scale photos with the same parallax would have a 

 factor of about 4.5 feet. Comparable preciseness in instrument and 

 skill in operator would result in spot heights with an error of 

 estimate approximately seven times as great. 



8/ These were enlargements to 1:31,500 from 6-inch F.L. , 1:40,000 

 contact scale photos flown for mapping purposes. 



_4- 



