70 ENGINEERING FOR LAND DRAINAGE. 



upon the contour map. The contours show points of 

 equal elevation over the entire field in such a way that 

 the surface slope is shown to the eye at a glance. 

 However, for ordinary drainage work the topography 

 of a field may be found in sufficient detail by methods 

 previously described which require less labor. A 

 knowledge of the elevation and location of the high 

 and low points in a field or district is usually sufficient 

 for the effective planning of a drainage system. 



