Reciprocal Leveling 



Correction for curvature and refraction, as well as for instru- 

 mental inaccuracies, can be mechanically overcome by reciprocal 

 leveling, which consists in finding the difference in elevation be- 

 tween two points by two sets of observations. 



Set up between the points and near to one of them. Take 

 readings on each rod with the bubble carefully centered. Set up 

 in the same relative position with respect to the second point and 

 take two more observations. The true difference in elevation is 

 the mean result between the differences in two sets of readings. 



In running level lines across rivers or estuaries, where the effect 

 of refraction is variable and considerable, this method has a special 

 application. Establish bench marks on each shore as shown in 

 Fig. 20.* With the instrument at B, take aB.S. on the rod A, then 

 a F.S. on rod C. The lower dotted line, representing a true 

 horizon, the computed elevation of B. M., C, will be too low by the 

 distance between the dotted line and the point b. 



Fig. so 



Witn tne instrument at C, the computed difference in eleva- 

 tion will also be in error by the amount a' V which is the algebraic 

 sum of the curvature and refraction correction. A mean of the two 

 sets of observations will be a true result if the two intervals, due to 

 curvature and refraction, are equal. To be certain that this influence 

 is not a variable quantity, the best practitioners have used two 

 instruments for simultaneous observations. 



If the regular type of Y-Level is used for this class of work, 

 the variable power eyepiece with magnifications ranging from x!8 

 up to x36 will be found desirable to accommodate the optical 

 properties of the instrument to atmospheric and light conditions. 

 If our Precision Y-Level is employed for this work, the inverting 

 type of telescope will always give the best attainable visual result 

 and a choice of oculars giving a range of magnification between 

 x33 and x46 is the best adaptability of available means to the 

 required purpose. 



* Sft article, by F. If. A'n,,f> * /v/v/>/ I sir ling in fr'rw }'ort with Coast 

 Surrey Level as made by the />' -nb Optical Co., Trans M*n. Enfrs. 



1913, pp. 75-151. 



37 



