62 A MANUAL OF TOPOGRAPHIC METHODS. 



The angles A to B, B to C, etc., read in this way may be computed as 

 in the first method, always combining the measure A to B with the immedi- 

 ately succeeding- measure B to A to eliminate twist. There is a theoretical 

 objection to this process of deriving angles founded on the fact that they 

 are not independent, but in secondary work this objection may be ignored 

 as of little weight. 



For the 11-inch theodolite and for the new 8-inch instruments made 

 by Fauth & Co., all of which read by micrometer microscopes, four (4) sets 

 of measures on as many different parts of the circle will be required; and 

 for the repeating theodolite six (li) sets of measures will be required, all 

 measures being made according to the programmes given above. 



Under ordinary circumstances and with due care in centering, angles 

 measured as specified above should show an average error of closure of the 

 triangles not exceeding 5". 



Under specially unfavorable conditions the number of sets of measures 

 should be increased, care being always taken to shift the circle so as to 

 eliminate periodic errors. 



The practice of starting the measurement of an angle or series of 



angles with the microscopes reading 0° and 180°, 90°; and 210°, etc., must 



be avoided; otherwise the errors of these particular divisions will affect 



many angles. In shifting the circle it is neither necessary nor desirable to 



180 

 have the new positions differ from the preceding one by exactly . A 



difference of half a degree either way is unimportant as respects periodic 

 errors, and it is advantageous to have the minutes and seconds differ for the 

 different settings. 



Field notes should be clear and full. The date, place, name and num- 

 ber of instrument used, and the names of observer and recorder should be 

 recorded at the beginning of each day's work at a station. The positions 

 of the instrument and signals observed should be defined either by a full 

 statement or reference to such in each day's notes. The time of observa- 

 tions should he noted at intervals to show that the instrument does not 

 stand too long between pointings. 



