THE TOPOGRAPHICAL SURVEY OF INDIA. 277 



always been found practicable to check by regularly 

 chained lines. There are, however, other modes of 

 testing plane-table surveys, and as these entail less 

 labor and expense in hilly and jungly tracts, and are 

 quite as effective if thoroughly carried out, they have 

 been adopted generally, while the measured routes or 

 check-lines have only been pursued under more favor- 

 able conditions. Colonel Thuillier states that "the 

 inspection of the work of every detailed surveyor in 

 the field has been rigorously enforced, and the work of 

 the field season is not considered satisfactory or com- 

 plete unless this duty has been attended to." 



The rules laid down to insure accuracy in the 

 survey are first, that the greatest possible number 

 of fixed points should be determined by regular tri- 

 angulation ; secondly, that the greatest possible num- 

 ber of plane-table fixings should be made use of 

 within each triangle; and, lastly, that eye-sketching 

 should be reduced to a minimum. If these rules are 

 well attended to, the surveyor can always rely on the 

 value of the work performed by his subordinates. 

 But all these conditions cannot be secured in many 

 parts of the ground allotted to the several topographi- 

 cal parties, owing to the quantity of forest-land and 

 the extremely rugged nature of the country. Hence 

 arises the necessity for test-lines to verify the details 

 or for some rigorous system of check ; and this is more 

 especially the case where native agency is employed. 



