LAYING OUT DRAINS IN THE FIELD. // 



ary lines of the field or farm in order to check the work 

 in drawing the plat or map. If a level with an attached 

 compass is used, these notes can be taken with little 

 additional effort and time, as use of the compass can be 

 easily acquired. The compass gives all angles referred 

 to the magnetic meridian for that place, which fact 

 makes the work more expeditious than any other known 

 method. If done with an instrument having only a 

 graduated circle^ some line of the field or farm must 

 be taken as a base, and the angle which the first 

 straight line of the drain makes with it be measured. 

 Then the angles which the several tangents of the 

 drain make one with the other should be measured, 

 being careful to record whether these angles are right 

 or left. The same methods of checking by measure- 

 ments as the chaining is done should be observed as 

 those described for compass work. 



A Common Datum, 



Every drain in the system should have distinct notes 

 by itself similar to the example given above for illus- 

 tration. All elevations should be referred to the same 

 datum, so that the difference of any two elevation num- 

 bers anywhere in the system will show the actual differ- 

 ence of elevation of the land. In other words, the 

 levels should all be connected. This can readily be 

 done by observing this simple rule. When levelling is 

 begun on any new line or branch take the first back 

 sight on some point whose elevation is recorded and add 

 it to the elevation of that point for a new height of 

 instrument. 



