DRAINAGE 187 



In some cases it is more convenient to find the height 

 of the instrument in its first position above the point 

 determined upon for the outlet of the proposed drain, 

 and then assume the datum plane 100 feet below the 

 outlet. Then all figures showing elevation of points on 

 the surface or the bottom of drains may be compared 

 with the outlet by merely subtracting the 100, which can 

 be done by inspection, saving calculation with pencil. 



Now, turning the telescope toward the proposed point 

 of outlet for the drain, where the rodman will place the 

 measuring rod, the disk is again moved until it is in 

 line with the eye and the cross hair in the leveling in- 

 strument. We find that the reading on the rod now is 

 9.43 feet. To find the height of this point of outlet 

 above " datum," we have only to subtract this reading 

 from the height of the eye at the instrument called 

 " height of instrument " above datum, thus 9.43 feet 

 from 105.32 feet equals 95.89 feet; that is, the outlet of 

 the drain is to be 95.89 feet above the imaginary level 

 plane. Next a measurement is taken on the surface of 

 the ground immediately above the outlet, which may be 

 called Station o, and this is found to be 5.82 below 

 the instrument, or 99.50 above the datum plane. These 

 measurements and the calculations from them are all 

 placed in the note book ruled in the form for field notes, 

 as in Figure 81. This form makes it practicable to carry 

 forward all the calculations and to make a permanent 

 record as the taking of levels proceeds. 



Now, proceeding to the next stake, Station I, there 

 being 100 feet between the stakes, the reading is taken 

 in like manner, and it is found that from the instrument 

 down to the hub at the surface of the soil is 5.54 

 feet; subtracting this measurement from the height 

 of the instrument, 105.32 feet, we find the height 

 of the ground at Station I to be 99.78 feet above 

 datum. The instrument is next turned upon the third 



