THE TOPOGRAPHICAL SURVEY OF INDIA. 247 



conducted with extreme care. In the present Keport, 

 for instance, we find that the discrepancies between the 

 common sides of the triangles in other words, the 

 discrepancies between the results obtained by different 

 observers are in some cases less than one-tenth of an 

 inch per mile ; in others they are from one inch to a 

 foot per mile ; and in the survey of the Cossyah and 

 Garrow Hills, where observations had to be taken to 

 large objects, such as trees, rocks, &c., with no denned 

 points for guidance, the results differ by as much as 

 twenty-six inches per mile. These discrepancies must 

 not only be regarded as insignificant in themselves, but 

 must appear yet more trifling when it is remembered 

 that they are not cumulative, inasmuch as the pre- 

 liminary triangulation is itself dependent on the great 

 trigonometrical survey. 



Let us understand clearly what are the various forms 

 of survey which are or have been in progress in India. 

 There are three forms to be considered : (1) The Great 

 Trigonometrical Surveys; (2) The Revenue Surveys; 

 and (3) the Topographical Surveys. 



Great trigonometrical operations are extended in 

 a straight course from one measured base to another. 

 Every precaution which modern skill and science can 

 suggest is taken in the measurement of each base-line, 

 and in the various processes by which the survey is 

 extended from one base-line to the other. The accu- 

 racy with which work of this sort is conducted may be 

 estimated from the following instance. During the 

 progress of the Ordnance Survey of Great Britain and 



