_^54 forestry Quarterly. 



mate the slope of the ground accurately without having definite 

 points of sight. The final resulting profile by this method of 

 sighting will be sufficiently accurate to- be of real practical value 

 in plotting the final map. 



The errors in profile by this method will never be too large to 

 destroy the value of the work. A profile drawn from eleva- 

 tions taken with an aneroid would at times show glaring errors 

 in slope which errors would be detected at once in the field by 

 the compassman with an instrument based upon a geometric 

 principle such as the Abney. Aneroid reading frequently will 

 show errors of fifty feet in descending an abrupt slope of 200 feet 

 in less than five chains. The compassman must make an all too 

 apparent error with the Abney in order to introduce this same 

 error into his profile. Yet the aneroid reading stands' and the 

 error remains uncorrected by the office corrections applied to re- 

 move the errors caused by weather and temperature changes. 



The compassman will usually be able to take the sightsi 

 illustrated. Snags and trees are usually found in bodies of dense 

 brush. Furthermore the compassman can obtain the heights 

 of the snags and low trees sighted upon with sufficient accuracy 

 by guess alone. If the situation calls for greater accuracy 

 in the determination of the height of the tree or snag the com- 

 passman can obtain the height of the snag or tree using the Abney 

 as a hypsometer. 



The compassman should always bear in mind that he is not re- 

 quired to do work of precision. He is' asked to exercise ac- 

 curacy in judgment and to cultivate obtaining practical accuracy in 

 results without paying attention to distracting details. Always 

 bearing in mind the nature of the final map the compassman will 

 be able to recognize the important topographic features and he 

 will lose sight of the insignificant points of relief. Then in tak- 

 ing his sight he will be able to sight parallel to the general slope 

 of the ground or avail himself of other means of sighting as 

 trees, shrubs, etc., to obtain the slope profile. His judgment and 

 ingenuity will improve and speed will come with practice. 



In order to do accurate work the adjustment of the instrument 

 must be checked before using it. Simple diagrams and instruc- 

 tions are shown in figures i and 2 explaining how to check 

 the adjustment of the level and how to adjust the instrument 



