5. Orientation should be accomplished by at least two bark sights 



and should be checked occasionally to guard against ac- 

 cidental displacement of the table. 



6. Three-point locations should be made by the three nearest 



available points checked, if possible, by a fourth, and the final 

 orientation should be made on the farthest visible station 

 whether or not it was used in the location. 



7. Errors in orientation should be corrected in the field, but errors 



of position with reference to long check sights are better ad- 

 justed in the offic; by slightly shifting the points to accord 

 with check sights. 



Details of Procedure in the Quadrilateral Method. 



Referring to Fig. 21 , first set the point a\ carefully over the corres- 

 ponding end of the base line. A plumb need not be used unless the 



o 



, A Edgc of planetable 



o 



Fig. ai. Various Methods of Graphic Base Extension after 



A, The Auxiliary Point Method ; B and C, Quadrilateral Method; 

 D, Triangle Method. Scale, 2 in. I mile. 



39 



