HYDROGRAPHIC SURVEYING 



125 



need not be very accurately located; but if they are located 

 accurately, they afford a means of checking the accuracy of 

 the angular measurements. 



4. By Transit and Stadia. In calm and smooth water, 

 the distance AC, Fig. 5, may be determined by observing a 

 o o 



FIG. 6 



stadia rod held in the boat at the same time that the angle 



CAB is measured. 



5. By Stretching a Rope from Bank to Bank of a Narrow 



River or Channel. The points where soundings are taken are 



marked by tin tags secured to the rope, as shown in Fig. 6. 



6. By Two A ngles Meas- 

 ured in a Sounding Boat 

 With Two Sextants. Three 

 prominent objects E, C, 

 and D, Fig. 7, such as 

 church spires, lighthouses, 

 etc., are located by deter- 

 mining the distances EC 

 = a, and CD = b and by 

 measuring the angle W. 



FIG. 7 



A sounding, as F, is theft 

 located by simultaneously 

 measuring the angles A and B with two sextants. Then, the 

 angles X and Y can be obtained by the formula 



a sin B 

 cotX = -+cotS, 



in which 

 and 



o sm 5 sm A 

 = 360-(W+A + B), 

 = S-X 



