THE DIAMETER TAPE 



229 



pers horizontally, with the beam in contact with the tree at the point 

 desired, usually at B.H. If measured with the tips of the calipers, 

 the errors resulting from false adjustment or warping are exaggerated. 

 If measured with the calipers held at an angle, the point measured is 

 probably above D.B.H. and correspondingly too small. If measured 

 below D.B.H., a large error results from the rapidly increasing diameter 

 of the tree due to stump taper. An average measurement 6 inches 

 below the desired point or at 4 feet will incur from 5 to 8 per cent excess 

 volume, depending upon the rapidity of the taper. 



Where the exact average diameter of a tree is desired, two measure- 

 ments must be taken at right angles and the mean recorded to ts inch. 

 In timber estimating, where large numbers of trees are measured, but 

 one diameter is taken, with no efforts made to determine the average 

 even on trees of eccentric cross sections since it is assumed that errors 

 incurred in this way are compensating. A precaution sometimes used 

 is to measure half of the trees in one cardinal direction, and the remainder 

 in the other (French). 



190. The Diameter Tape. The irregularity in the form of trees, 

 both as to cross section and bark, makes it practically impossible to 

 obtain consistent results in two successive measurements of diameter 

 of the same tree 

 with calipers even 

 when the mean 

 diameter is taken 

 as above indicated. 

 For permanent 

 records on plots 

 to be subsequently 

 measured for deter- 

 mination of growth, 

 consistency in 

 diameter measure- 

 ment is absolutely 

 required. 



For this purpose it has been found that the diameter tape must be 

 substituted for calipers. The graduations on the diameter tape are in 

 inches of diameter, each inch equal to 3.1416 inches in girth. In theory, 

 the measurement of the circumference of a tree gives a plus error when 

 compared with the actual mean diameter. Actual tests at the Fort 

 Valley Experiment Station by Scherer on one hundred trees showed 

 that the excess in diameter from tape over caliper measurement was 

 2 per cent, but the consistency of two successive tape measurements 

 as compared with successive caliper measurements showed that the 



Fig. 41. — Tape for measuring girths and diameters. 



