84 HYDROGRAPHICAL SURVEYING [chap. iv. 



The main triangles should be as large as possible. The 

 fewer triangles there arc, the fewer are the chances of errors 

 of observation. 



IVIAKING A MAIN STATION. 



Choice of Observing angles at a station is technically called " making " 

 ®^°' it. Let us suppose a surveyor making a first station, probably 

 one of the base stations. 



He has been previously furnished with a list of the main 

 stations visible from him, and has been told how many times 

 his angles to them are to be repeated. He has also received 

 instructions about the secondary stations and minor marks, 

 if any have been selected and marked. 



Having levelled the theodolite, the first thing is the choice 

 of an object from which to measure all the angles, which is 

 called the zero. 



A zero should be, if possible, another main station. It must 

 be at some distance, but not so far as to be easily obscured 

 on a hazy day ; well defined ; so placed that the rays of the sun, 

 when it moves from the position in which it happens to be when 

 the station is commenced, will not obliterate it. It should be 

 a fixed object — i.e., not likely to be removed, or to tumble down, 

 and not so high as to be covered with clouds, as a mountain 

 peak. 



A great deal of trouble is given when a zero has to be changed, 

 or when on a subsequent visit to a station the same zero cannot 

 be used. Attention to the above-mentioned points is, there- 

 fore, of importance. 



The bearing of the zero by the theodolite compass should 



always be entered in the book. 



Observe The zero fixed upon, and the theodolite directed upon it, 



AnRks observe the main angles, or those to the main stations, first, 



first. repeating them the required number of times, by either of the 



two methods described under " Theodolite." 



These completed, observe the secondary stations a sufficient 



number of times, as well as all marks and conspicuous objects. 



It is important to remember that the position of the sun 



has a great effect on the visibility of objects, and therefore 



that those stations and objects on which the sun is shining 



