SOME NEW ASPECTS REGARDING THE USE OF THE 



FOREST SERVICE STANDARD (GRADIMETER) 



HYPSOMETER 



By Hermann Krauch 

 Forest Examiner, U. S. forest Sermce 



While engaged at the remeasurement of sample plots in the fall of 

 1916, the writer found that the new measurements of tree heights were 

 in many instances lower than those secured five years previously. An 

 attempt was therefore made to determine the reason for these dis- 

 crepancies. The source of error was at first ascribed to the fact that 

 different types of hypsometers were used during the respective periods 

 of measurement. The same deduction has also been made by other 

 investigators.^ 



The use of incorrect horizontal distances in taking readings (es- 

 pecially where steep slopes are involved) was also thought to be a 

 factor. But careful investigation indicated that the condition which is 

 most apt to cause discrepancy between two sets of readings is the 

 variance of trees from a vertical position.^ In other words, readings 

 taken from dififerent points are not the same for the reason that hori- 

 zontal distances (top of tree considered) are different. The following 

 diagram (figure i) will make this point clear. 



While the readings from points A and B may have both been taken 

 at distances measured along* a horizontal line exactly 100 feet from 

 the base of the tree, the distances actually involved, in so far as the 

 use of the hypsometer is concerned, are different, being 104 feet and 

 96 feet, respectively. 



The degree of discrepancy between readings taken from different 

 points, moreover, increases in direct proportion with tlie increase in 

 heights of trees measured. Two factors affect this : 



I. A proportional increase in horizontal distances, considering that 

 these distances are measured from the base of the tree, whereas the 

 position of the top is actually involved. 



' See article — "Comparative Test of Klaussner and Forest Service Standard 

 Hypsometers" — Douglas K. Noyes, October (1916) Proceedings. 



^ This was tested repeatedly by taking readings of trees from different direc- 

 tions. Wherever great discrepancies occurred it was found that the trees were 

 not standing vertical. This condition is not readily perceived without the use 

 of a plumb-line, but the fact is that there are few trees which stand absolutely 

 vertical. 



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