TI3IBER ESTIMATING. 83 



their class, whether small, medium, or large. When the height of a 

 tree is measured, the diameter also is noted. After these height 

 measurements have been made a curve is constructed from which 

 a table may be made to show the average height of trees of any 

 diameter. This is done in the following way: On a sheet of cross- 

 section paper (fig. 3) let the vertical lines represent successive 

 diameters at 1-inch intervals, and the horizontal lines successive 

 heights at 1-foot intervals. Assume, for example, that the follow- 

 ing measurements were taken for Loblolly Pine: 



Diameter in inches 10 15 19 24 



^ Height in feet 75 89 99 101 



Mark on the cross-section paper the point where the vertical 

 line running from the diameter point 10 meets the horizontal line 

 running from the height point 75. Mark the points of intersec- 

 tion for the other diameters and heights in the same way. Then 

 draw a regular curve through or as near the points as possible 

 in the way shown in fig. 3. The height corresponding to any 

 diameter may then be read off from this curve. Thus, to find the 

 height of a 16-inch tree, note the point where the vertical line 

 running up from the 16-inch point meets the curve; then from this 

 point of intersection follow the horizontal line to ascertain the 

 height. In the example given the average height of a 16-inch 

 tree is 86 feet. 



This method is used to find the contents of sample acres in the 

 following way: Make a table of four columns. In the first column 

 ^place the diameters; in the second column the number of trees of 

 each diameter given in the first column; in the third column the 

 average height of trees of each diameter, these average heights being 

 obtained from a curve such as has been described; in the fourth 

 column the contents of an average tree from a volume table; in 

 the fifth column the total contents of all trees of each diameter. 

 Then add the fifth column, and the result will be the total con- 

 tents of the sample acre. 



