62 ENGINEERING FOR LAND DRAINAGE, 



Water-course as a Base, 



A second method especially applicable to district 

 work is to use the main water-course as a base and re- 

 fer all other lines to it. In this case the line should be 

 measured, stakes set at the angles and levels taken. 

 Ordinates may be run from this line and in such direc- 

 tions as the judgment of the engineer may dictate. 

 The object is the same in either case, viz., to get the 

 course and slope of the natural depressions, to find the 

 water-shed lines and the area of the drainage-basin. 



Method by Central Base Line. 



Many tracts of land have such irregular boundaries, 

 with no well-defined outlet stream or other prominent 

 features, that the methods previously given for making 

 a preliminary survey will not be as easy of application 

 as the one to be described. Run a central base line 

 through the longest dimension of the field or plantation, 

 setting stakes and solid hubs at distances of 400 feet. 

 (See Fig. 12,) These are to be used as permanent 

 stations in all subsequent work of a preliminary nature, 

 and the line should be described from a compass bear- 

 ing. Levels should be taken upon each hub and used 

 as bench-marks. All of the low and high points may 

 now be sought out by inspection or with the aid of the 

 level and marked by a stake and hub. Find elevation 

 of these points. Take a compass bearing and measure 

 the distance from these points, to the nearest station 

 on the base line, or if a circle without compass needle 

 is used, set up at the nearest station and turn off the 



