66 AGRICULTURAL ENGINEERING 



which will show definitely the lay of the land. It is not safe 

 for even the most experienced to estimate slopes by the 

 naked eye. 



Map of the Preliminary Survey. A sketch or map 

 should be made indicating the location and elevation of the 

 low and wet areas in the land, and also the watershed. In 

 some cases where the land is quite flat it is desirable to take 

 levels at regular intervals over the entire tract, and, perhaps, 

 to prepare a contour map as explained in a previous chapter. 

 With this information it is possible to lay out the drainage 

 system, if conditions show that a practical system is possible. 



It is desired to lay special emphasis upon the importance 

 of this preliminary survey. The quite common practice of 

 laying tile largely by guess, without a consideration of the 

 land area to be drained or the capacity of the tile, cannot be 

 too severely criticised. The large amount of insufficient 

 and unsatisfactory drainage to be found everywhere is silent 

 testimony to the statement that tile drainage must be done 

 carefully and intelligently. 



QUESTIONS 



1. What is the purpose of a preliminary survey? 



2. Why should a drainage engineer be employed on important 

 work? 



3. What is the difference between surveying and drainage engineer- 

 ing? 



4. What should be included in the preliminary survey? 



5. Why should the subsoil be investigated? 



6. To what extent should an instrument be used in a preliminary 

 survey? 



7. What should be included in the map of the preliminary survey? 



