MEASUREMENTS 15 



passing through heavy vegetation there is a tendency for 

 several of the links to double back over themselves in a 

 knot, thus shortening the chain 2 or 3 feet. Consequently 

 the chainmen should be continually on the alert to see that 

 their chain is properly stretched and straight. 



Length and zero point. Measuring equipment made by 

 different manufacturing firms varies somewhat in the details 

 of graduation and in the location of the zero point. Chains 

 and tapes of special length are frequently used. In begin- 

 ning a series of measurements with any kind of chain or 

 tape the chainmen should first determine the length of the 

 unit and the location of the zero point. An examination of 

 the unit will determine the length, and the zero point on the 

 end foot can be determined by comparison of that link or 

 foot with a link or foot near the center of the chain or tape. 



Errors in lineal measurement. Two sources of error must 

 be guarded against in all lineal measurements. One is the 

 small errors that continuously increase, such as expansion, 

 worn links, and the like. The other is the error resulting 

 from individual blunders by men making the measurements. 

 The blunders can usually be traced to one of the following 

 causes : loss of a tally pin, which means that the distance is 

 shortened one unit length; loss of a tally, which means an 

 error of 10 unit lengths; the wrong reading of a fractional 

 tape, which means an error of 1 to 50 feet in a 100-foot 

 chain or 1 to 33 feet in a 66-foot chain; failure to set the pins 

 marking the chain lengths vertical, which 

 means an error of 1 to 3 inches in each 

 chain; not pulling the tape in straight 9 \ 



lines, which gives an error #, -4*^ FlG - 2 - In 



increasing with the dis- j ^S^ measurement of 



...... f . c, -4>^ land on sloping ground 



tance which the chain may ; ^^ [he distance AB is equal to 



be Off the line ; not holding A^ the sum of cd, ef,gh, and kB. 



the tape level on sloping ground, which means measuring 

 the hypotenuse of a triangle when the base should be 

 measured; not carrying the end of the tape straight to the 



