CHAP. iii.J CHAINED BASES 67 



calculated between the terminal points should be just as correct 

 as if it had been measured direct. 



It is seldom that a chained base can be found, even for a Extension 

 small plan, long enough to plot from directly, i.e., the 

 measured length when protracted on the paper would be 

 generally so short, that by placing that on the sheet first, 

 and making it the starting line, errors would be sure to creep 

 in, in increasing the size of the triangles, any httle error being 

 multiplied. It therefore is usually necessary to extend the 

 base, as it is termed. 



This consists simply in calculating a sufficient number of 

 triangles, conveniently arranged, to obtain a side long enough 



.c 



Fig. 10. 









to form a good start, so as to plot inwards as much as possible, 

 when any little errors will be diminished, instead of increased. 



As a commencement of this process, the base to be measured 

 should, if possible, be placed so that there are two stations, 

 one on each side of it, wliich can be used for the first triangles 

 and consecjuent extension of the base. 



Here, Fig. 10, A B is the measured bas, C and D the two 

 first stations. Angles are observed at A, B, C, D. The other 

 two sides in the triangles A B C, A B D being found, C D can 

 be found in both the triangles A C D, B C D, wliich will check 

 the result, and C D will be the extension of the base for further 



triangulation. 



5—2 



