FORESTRY QUARTERLY 



Vol. XIII December, 1915 No. 4 



THE PROFILE ALIDADE 



AN INSTRUMENT THAT MAY PROVE USEFUL IN 



MAPPING STRIP LINES 



By S. B. DetwilEr 



The recent discussions in the Forestry Quarterly of the use 

 of the Abney hand level for obtaining elevations in strip mapping 

 made it plain that the instrument has disadvantages as well as 

 advantages for this purpose. Some of the disadvantages of 

 a hand level are the impossibility of securing a close reading, the 

 time required to secure a proper adjustment of the bubble and to 

 transfer the data to the notes, and the eye strain caused by sight- 

 ing through a fine aperture which cannot be held absolutely 

 steady. In running about 40 miles of levels with the Abney, of 

 which the writer had charge, it was found that when the instru- 

 ment was lashed firmly to a camera tripod with a ball and socket 

 head, readings to half a degree could be made when the observer 

 used the instrument for some time and kept in training. By 

 having the instrument held steady much of the eye strain was 

 relieved, and on resurvey the readings checked almost exactly. 

 This method of using the Abney level is fairly convenient in 

 leveling base lines and streams, but, of course, is not suited for 

 use in strip survey mapping. 



It is evident that the force of gravity will do the same work 

 as a level. Since a plumb line is at right angles to a horizontal 

 line, if a level bubble is fastened at right angles to the line of 

 gravity in a pivoted pendulum, the bubble should be centered 

 when the pendulum is at rest. Therefore, if a pendulum was 

 fastened to the pivoted arc of the Abney level and the reflection 

 of the degrees on the arc thrown on the mirror instead of the 

 bubble reflection, the operation of the level would be automatic. 

 This idea is applied in the construction of the Forest Service 

 standard hypsometer. 



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