GRAVITATIONAL METHODS 411 



deviation from the drift line occurred at the second trial and is 0.10 milli- 

 gal. This accuracy is somewhat better than necessary for obtaining useful 

 gravity maps. 



The error caused by inaccuracy of cahbration can partially be tested 

 by determining the error of closure of closed loops of gravity differences. 

 If the scale readings are converted into gravity values by means of an 

 incorrect calibration curve, which might be the case if the sensitivity of 

 the instrument changed during the course of a survey, errors of closure 

 will appear. There is, however, also the possibility that an initially incorrect 

 calibration might give proper closure, and yet all the measurements might 

 be too large or too small by a constant percentage. This possibility may be 

 detected in several ways. One method is to make observations at different 

 known elevations in a building. (Compare p. 389.) Another is to check the 

 gravimeter against gravity differences known from pendulum observations. 

 Gravimeters are considered satisfactory if such tests indicate calibration 

 errors of less than two per cent. 



Errors caused by incorrect latitude and elevation corrections can be 

 reduced to practically negligible proportions by surveying with sufficient 

 accuracy and by taking care to use the proper elevation correction constant. 

 It is practicable to establish the latitude of each station to within about 

 one-twentieth of a mile. Thus the error in the latitude correction is about 

 five hundredths of a milligal. It is also perfectly feasible to require closure 

 of level line loops to within one foot, so that the error from incorrect ele- 

 vation is not greater than about seven-hundredths of a milligal. 



Sample Field ISotes. — The various corrections applied to gravity meter 

 readings are shown in the sample set of field notes on pages 412 and 413. 

 The forms employed for field notes differ with each organization. Those 

 illustrated here use two separate sheets. One of these is termed the 

 Gravity Computation Sheet, and is used to record instrument observations 

 and related data. The other, called the Correction Sheet, provides for the 

 necessary corrections, which are summed up and the final correction trans- 

 ferred to the computation sheet. 



The gravity computation sheet has 15 columns, the first three showing 

 station number, time of observation, and instrument reading, in that order. 

 The first reading at the base station in the sample set is 5-1360. The 5 

 refers to line five of a set of 10 lines in the moving scale of the meter 

 used (Brown Gravity Meter No. 21). The position of this line on the 

 fixed scale is 1360 scale points, or scale divisions. 



The scale of this instrument is not quite linear, and the necessary cali- 

 bration is achieved by adding a weight to the moving system, to determine 

 the difference in scale divisions it causes. In this case, at the start of the 

 survey, addition of the weight made a difference of 610 scale points. This 

 calibration was made again at the end of the day's run, at which time there 

 was a difference in reading of 600 scale points. 



