38 CIECULAE 3 3 3, U.S. DEPARTMENT OF AGRICULTURE 



ball-and-socket attachment of a compass. A notch is then cut around 

 the top of the handle, about an inch from the socket seat, to permit 

 tying the ball and socket or the hypsometer to the staff with a stout 

 cord. The spike, with its thimble, should be removable. Properly 

 fitting the thimble, or using a screw to hold the thimble firmly in 

 place, makes it possible to remove the spike for convenience in pack- 

 ing. If a step or foot-rest attachment is fixed to the handle about 

 2 or 3 inches from the thimble of the spike, it is possible to force the 

 staff firmly into the ground by foot-pressure. The staff then makes 

 a very firm rest. 



Hypsometers should be kept clean and should be maintained in good 

 working order at all times. Before being used, a hypsometer should 

 always be thoroughly tested to make sure that it is in perfect adjust- 

 ment. It should be tested each day it is used. 



Height measurements should be taken in the same general direc- 

 tion on a plot, and the field notes should clearly indicate this direction. 

 On sloping ground, measurements should consistently be taken from 

 the uphill side. Under this practice some of the inaccuracies due to 

 lean are eliminated, as the lean of each tree is in the same general 

 direction with reference to the observer. 



The hypsometer should in general be used at a distance from the 

 tree that is greater than the tree's height ; in taking heights down a 

 slope, it may be used at an appreciably shorter distance. In dense 

 stands of uniform height, or in any stands of hardwoods during the 

 leafy season, it is often exceedingly difficult to use a hypsometer 

 with any great degree of reliability. Height measurements of hard- 

 woods should invariably be made when the leaves are off the trees. 



Where trees more than 125 feet tall, and only a few trees, are to 

 be measured periodically, the use of the transit may be justified. In 

 such cases, permanent hubs should be set up and the same trees 

 should be measured from these each time. This system tends to save 

 time and expense, and to eliminate errors. 20 If a transit is used, 

 the reading for the tip of the tree and that for its base should both 

 be recorded, so that errors in calculating height may be checked. 

 Care should be taken to give each reading the proper sign (plus or 

 minus). As the base of the tree frequently is an indeterminate point 

 or is hard to distinguish through an instrument or in brushy stands, 

 the top of a measuring stick held against the tree may be used as the 

 base and the length of the stick added to the calculated height of 

 the tree, or the breast-height point may be used as the base. 



The distance from the point of observation to the tree may be 

 measured with a tape, with a range finder, or with a telescopic alidade 

 and stadia rod. The tape is the most accurate, but in some stands 

 it may be impossible to make successful use of any instrument other 

 than the range finder (61). On sloping ground, the use of a slope 



20 The Revised Instructions for the Establishment of Permanent Sample Plots of the 

 Canadian Forest Service (mimeographed, Ottawa, April 1929) suggest that the height 

 of each tree be measured from the base of some other tree on the plot, and a note 

 "from tree no. — " recorded with each height. These instructions proceed: "Where 

 a map has been made, the distance from the instrument or base tree to the various trees 

 measured may either be scaled on the map or measured by tape. As the former method 

 assumes that the map is accurate, a small check should be made to determine if the 

 map distances between trees coincide with the actual distances on the plot. Errors 

 in this check should be less than one foot." 



