CHAPTER IV. 

 LEVELLING AND TOPOGRAPHY. 



Levelling. 



Levelling is the art of finding how much one point 

 is higher or lower than another. 



A Level Line is one which is perpendicular to the 

 direction of gravity. A Lcvellmg Lzsirtmient is any in- 

 strument by which a level line can be accurately deter- 

 mined. 



A Datum Plane, or ''Datum, " is the initial plane or 

 point in the plane from which all heights or elevations 

 are computed. 



Tlie Elevation of a point is its height when referred 

 to datum, or its vertical distance above or below datum. 



A Levelling-rod is a graduated staff for measuring 

 the distance from the line indicated by the instrument 

 to the point whose elevation is desired. A Target Rod 

 has a sliding disc which is moved by the rodman to the 

 position indicated by the man at the instrument. The 

 rodman is expected to call off the reading of the figures 

 as indicated by the position of the disc. A Speaking- 

 rod is one graduated with such distinctness that it can 

 be read by the instrument man with precision. The 

 speaking rod is preferable for use in all drainage work. 



