DRAINAGE IQI 



" Back sight " and " fore sight " or " plus sight " and 

 "minus sight/* In the tabulated statement, Figures 81 

 and 82, the measurements from points of known heights 

 up to the instrument are called back sights, or some- 

 times plus sights; that is, we add the height from the 

 surface up to the line of sight through the instrument to 

 the known distance above datum of the surface where 

 the leveling rod stands in order to get the new height of 

 the instrument. 



There is a column in the table headed fore sight, some- 

 times called minus sight. In this column are placed 

 the measurements from the height of the instrument 

 down to given stations, the height of which it is desired 

 to know. Since the instrument is always higher than 

 the point we wish to measure, we always subtract the 

 measurement from the height of the instrument down to 

 the point in question so as to determine the height of that 

 point above datum. These measurements, which are to 

 find the height of the surface of the ground, are properly 

 termed minus sights, to be subtracted from the height 

 of the instrument, as distinguished from the plus sights, 

 which are added to the known heights of given points, 

 which are always used to find the height of the instru- 

 ment. Back sights are often abbreviated B. S. and fore 

 sights F. S., in the tabulated notes. 



How to use measurements of levels. Now that the 

 levels of the successive stations along the proposed line 

 of the ditch have been determined, a method of using 

 them is necessary. Where there is considerable fall, 

 very little surveying or calculation is needed since an 

 experienced person can determine the depth the ditch 

 should be at each station by merely inspecting the 

 figures and the land. In cases, however, where the pro- 

 posed drain is long, where the grade is slight, or where 

 slight elevations requiring deep ditches necessitate con- 

 siderable expense, or where the connection of lateral 



