352 forestry Quarterly. 



the crew. Topography will influence the speed of the com- 

 passman, and timber cover will determine the speed of the esti- 

 mator. Where there are many species the estimator will work 

 slowly. Where there are only a few species or only one the 

 estimator will work fast. Thus where both topography and 

 timber cover conditions favor speed in the work of the recon- 

 naissance crew as is the case in uniformly sloping Yellow pine 

 country, conditions are very suitable for the use of the aneroid. 

 The strips can be run with speed and frequent comparison with 

 known elevations will correct the errors in the elevations taken 

 along the strip. 



3. Weather conditions must be favorable to the use of the 

 aneroid. Wilson states : — "It frequently happens as on the ap- 

 proach of a storm or change from stormy to clear weather that 

 atmospheric pressure will change in a few hours by over an inch. 

 This means an apparent change in elevation at one place of 

 over 1000 feet or more." In winter cruising on the Crater Na- 

 tional Forest the corrections of the great aneroid variations caused 

 by the unsettled weather conditions will introduce errors of 

 over 300 feet in the final corrected results. It has been found 

 almost impossible to correct these big errors caused by the daily 

 weather variation in the winter time. The aneroid is most suit- 

 able only when the weather conditions are settled. 



The Abney Hand Level. 



Realizing that the aneroid is liable to introduce errors in the 

 horizontal location of contours when that instrument is used 

 under unfavorable circumstances, the following methods have 

 been outlined in order to substitute a geometric system of ob- 

 taining vertical control. It should be borne in mind, however, 

 that the trigonometric methods here mentioned should not be 

 used with the object of doing work of high precision. The sole 

 aim of the use of these methods is to bring the work of obtain- 

 ing vertical control within the proper limits of accuracy which 

 limits can not be attained with the aneroid under prevailing 

 unfavorable conditions. If these methods of using the Abney are 

 used with a clear conception of the degree of precision to be 

 attained the mapper will not waste time in attempting to do 

 work of too great refinement. 



