THE JONSON FORM POINT METHOD 249 



The instrument consists of a graduated scale or straight-edge. For determining 

 merely the form quotient the actual diameters need not be ascertained but only 

 their proportion or relation. The two measurements are taken by eye, holding the 

 horizontal scale at arm's length {Ac and Ah) for each reading. The principal 

 error to be guarded against is failure to secure the horizontal line of sight and the 

 corresponding distance, which will result in correspondingly large errors in reading 

 the proportional diameters. Failure to select the right point on the tree, provided 

 a definite point is selected and the method otherwise properly applied, incurs only 

 the error due to difference in taper between the point measured and the point 

 desired, which depends on rapidity of taper. 



This simple method should be of great assistance both to practical .woodsmen 

 in determining upper diameters, and to foresters desirous of testing the form quotient 

 of trees in order to ascertain the applicability of volume tables based upon principle 

 of form factors. 



203. The Jonson Form Point Method of Determining Form Classes. In con- 

 nection with his studies of the form of trees and form quotients, Tor Jonson has 

 evolved a method for determining the form class of standing trees without the 

 necessity of measuring the upper diameter or the form quotient. 



This method consists in locating a point on the stem of the tree, which he terms 

 the form point. The percentage relation which the height of this point from the 

 stump bears to the total height of the tree, he claims, bears a consistent relation 

 to the form quotient, and by means of a table showing these relations the form 

 quotient and form class of the tree may be determined. 



Mr. Jonson describes the method as follows: 



The shape and position of the crown has been found to be the most dependable 

 and useful indication of different tapers and form classes. This is connected with 

 the bole's function to carry and steady the crown, especially against the breaking 

 forces of the wind, and it has been found that in the building of the bole only 

 enough material is used to make it equal in strength to the force of the winds. It 

 may therefore be said that it is the strength of the winds that determines the 

 necessary dimensions of the trunk, and as the force of the wind is generally applied 

 to the crown of the tree, it will be found that its weight, shape and position indirectly 

 influence the size and taper of the trunk. 



While estimating, the D.B.H. is measured with caliper and the taper is then 

 determined through finding by ocular means the form point, i.e., the point where 

 the pressure of the wind is apparently concentrated which is usually the geomet- 

 rical center of the crown. By sighting at this point and at the same time at the 

 base and tip of the tree over a stick, approximately 30 cm. long, divided into 10 

 equal parts (Christen's hypsometer), the height of the form point can be easily 

 found expressed in per cent of the total height. This form point can then be 

 lpoked up in the table giving the form point heights which are characteristic for 

 each form class. The higher the crown is placed, the less the taper and the more 

 cylindrical the form, and conversely, the lower the crown extends, the more rapid 

 will be the taper and the poorer the form. 



When, as is often the case, the estimating is based on diameter outside bark, 

 the difference which is caused by variable thickness of the bark must be taken 

 into consideration. The spruce, fir and other species with thin even bark show 

 no difference in form when measured inside or outside bark, for which reason the 

 given normal form point heights give the form with, as well as without, the bark 

 for these trees. 



White birch, larch and others, but especially the pine, show great reduction in 

 form when measured with bark, for which reason the form quotient outside bark 



