68 HYDROGRAPHICAL SURVEYING [chap. iit. 



Of course this desired convenience will not always be found, 

 but it is a thing to look out for. 



It is by no means necessary to measure a long base, provided 

 that convenient triangles can be found for extending the base 

 by calculation. If the angles of these are of the necessary 

 number of degrees, and they are carefully observed with theo- 

 dolites, a short base, measured on flat smooth ground, will give a 

 truer result than a longer one measured over inequalities. With 

 a sextant survey it will be well to have as long a base as possible. 

 Planting The ground having been walked over to ascertain its fit- 

 m^as^ur-"'^ ness, and the base stations (the ends of the base) being so 

 ing by. placed that they see as much as possible on all sides : set 

 up the theodolite at one end, and at the other a flagstaff or 

 another theodolite, and let a man plant staves (boarding pikes 

 make good ones) exactly in line between the two stations, 

 giving him the position for the first two or three, by looking 

 through the theodolite directed to the other station. After 

 these are in place, he can plant the others in line by guiding 

 himself by them. 

 Method of Having the staves placed and in line, begin to measure from 

 measur- ^^^ ^^^ j£ ^^^ persons are to measure, begin from opposite 

 ends. A man is required for each end of the chain. The 

 man at the foremost end of the chain carries ten pins, and 

 the surveyor attends with his book to see the chain fairly 

 placed in line between the staves, and to note down each 

 length of chain measured. Do not let the men stretch the 

 chain too tight, but it must lie straight on the ground between 

 the two ends. 



The chain being down for the first length, measuring from 

 under the centre of the theodolite, put a pin in the ground, 

 at the foremost end, inside the handle, and touching the flat 

 side. ]\Iake a mark in the note-book, and walk on together, 

 the man at each end lifting the chain as much as he can, 

 until the hindermost comes to the pin. He must then place 

 the outside of his handle so as to touch the pin. Another pin 

 is put in at the foremost end inside the handle, the second 

 note made in the book, the first pin taken up by the hinder- 

 most man, and on you go. 



The lengths are best noted by strokes, crossing every fifth 

 over the four, as in ordinary tallying. 



