CHECKING THE ACCURACY OF VOLUME TABLES 189 



Top diameters used — by diameter classes if variable. 

 Method used in constructing table, 



a. Based on measurements at fixed intervals. 



b. Based on measurements of logs as cut. 



c. From tables of taper or form (Chapter XV). 



d. From form factors (Chapter XVI) 

 Author, and year of preparation. 



The basis of classification of volumes, as to height and diameter, 

 is shown in the table itself. But tables based solely on diameter will 

 have their value increased if the average heights used in constructing 

 the table are also shown (§ 162). 



161. Checking the Accuracy of Volume Tables. Volume tables 

 make no pretense of giving accurately the volume of single trees (§ 121). 

 If the average values given coincide with the average of the volumes 

 of the trees to be measured, the table is accurate for the purpose in hand. 



But, although applied correctly (§ 158) volume tables will grve 

 inaccurate results, first, if the table itself is inaccurately made and does 

 not give correctly the volumes of the trees from which it was constructed, 

 second, if the trees to be measured average greater or smaller volumes 

 for given diameters and heights than those given in the table, on account 

 of fuller form or vice versa. 



Volume tables made in one locality may be serviceable in other 

 regions, covering the entire range of a species. If the estimates are 

 made to conform with the top diameters and log rules used in the table 

 the only possible variation in volume from such tables is that of average 

 form, and variations due to this factor can be determined without 

 constructing an entirely new table (§ 171). 



To check the accuracy of construction of a table, the basis in trees 

 is first considered. Tables based on from 500 to 1000 trees or more 

 are regarded as fairly reliable, while if fewer trees have been used the 

 table is open to question. The total actual volume of the trees used 

 in constructing the table can be checked against the total volume of 

 the same trees figured from the table. This gives a basic check which 

 may, however, conceal compensating errors. The average volume of 

 the trees in each diameter and height group may then be checked 

 against the tabular values in the same way, and the errors recorded 

 in terms of per cent. These errors should compensate. A still more 

 accurate check is to record the divergence in volume of each tree from 

 the tabular volume and total the per cents of error plus and minus, 

 which should compensate. Or, the plus and minus errors may be 

 plotted to detect any trend towards high or low values at one end or 

 the other of the curves. 



