SAMPLE PLOTS IN" SILVICULTURAL RESEARCH 39 



or topographic tape with an Abney makes for rapid work. With an 

 improved Abney, slope distance can be converted into horizontal 

 distance with practically no effort (66) . 



Distances should be measured from the rear sight of the hypsometer 

 to the center of the bole at average ground level, or to the breast- 

 height point. Measurements to the breast-height point tend to be 

 more consistent. The man holding the tape at the tree can indicate 

 the breast-height point when it is not clearly seen by the instrument 

 man. In measuring leaning trees an effort should be made to deter- 

 mine the exact vertical projection of the tip, and the distance from 

 the hypsometer to this point should be taken as the distance to the 

 tree. 



If the top of a tree is hard to see, or if the top is round and the 

 tip indistinct, it may be necessary to make 2 or 3 height readings 

 with the hypsometer from different directions in order to get a 

 reliable figure. 



For height measurements made with the hypsometer in connec- 

 tion with studies in which height growth is to be followed with 

 care, the following standards of accuracy are suggested: For trees 

 less than 60 feet tall, height should be determined within one-half 

 foot, except in the case of trees (such as round-topped hardwoods 

 or flat-topped conifers) the tips of which cannot be accurately de- 

 termined; for trees between 60 and 125 feet tall, height should be 

 read within the nearest foot; for trees more than 125 feet tall, 

 height should usually be read within 2 feet, except in dense stands 

 In the majority of stands, such accuracy may be impossible. 



For trees less than 18 or 20 feet high, a measuring rod may be 

 used. This may be an extension rule designed like a leveling or stadia 

 rod, such as is shown in Figure 2 (69, 80), or a rod on which colored 

 bands represent different linear units. Extension rods with a pulley 

 arrangement have been developed for measuring trees as tall as 35 

 feet. These work less well, however, at heights of more than 30 

 feet. 



In most studies, only normally shaped trees of the crown class 

 concerned should be measured for height. Where it is possible to 

 make a choice, leaning trees should not be measured. Occasionally 

 it is necessary to measure an abnormal tree for height; when this 

 is done, an accurate description of the tree should be recorded. 



For how many trees height measurements should be made on an 

 even-aged plot depends on the use that is to be made of the meas- 

 urements. In some cases, as in studies of height growth and 

 volume production, the heights of all the trees may be needed. If 

 the height data are to be used in determining plot volume alone, 

 only enough measurements for a smooth diameter-height curve are 

 needed; but this curve should be well substantiated by a series of 

 strong points. In this latter case a summary of the diameter meas- 

 urements on the plot will show what height measurements are 

 needed, or heights may be plotted over diameters as they are taken 

 in the field until there are enough points to give a good curve. If 

 heights of individual trees are to be followed throughout the life 

 of a plot, more should be taken than are needed at first, so as to 

 allow for the cutting or death of some of the trees. The general 



