STADIA AND PLANE-TABLE SURVEYING 95 



constant being disregarded; but, for more accurate work, the 

 constant usually taken is 1 ft. 



To verify the constants, a line from 400 to 800 ft. is run on 

 level ground and careful rod readings are taken at intervals of 

 50 ft. Let Rz and Ri be two stadia readings taken at the 

 respective distances dz and di; then, 

 dz-di 

 S ~Rz-Ri 

 and i= diRz- zRi 



Rz-Ri 



Several pairs of readings and their corresponding distances 

 are substituted in these formulas, and the mean of all the 

 resulting values of 5 and * is calculated. 



EXAMPLE. Determine the stadia and the instrument con- 

 stant from the following data: 



Distance Measured Rod Reading 



Feet Feet 



50 .488 



100 .988 



200 1.988 



300 2.991 



400 3.986 



SOLUTION. Take 50 ft. for the value of di and 100 ft.; 200 ft., 

 etc. successively for the values of dz, and apply the preceding 

 formulas for 5 and i. For the first pair of observations: 

 100-50 



.988 -.488 





SOX. 988 -.488X100 



and i = = 1.200 ft. 



.988 -.488 ' 



The other values are figured in a similar manner and the 

 whole is tabulated as follows: 



5 i 



100.000 1.200 



100.000 1.200 



9 9.8 8 1.2 5 b 



100.057 1.172 



4 ) 3 9 9.9 3 7 4 ) 4.8 3 



s 99.984 = s 1.208 = * 



