aa THE IMPROVKMKXT OF RIVERS. 



and level parties, and one hydrographic party, each being provided with the necessary 

 skiffs, etc. A line is run along each bank and marked with stakes and bench-marks 

 in the usual way, connecting angles being turned even' two or three miles on stations 

 on the line on the opposite bank, for the purpose of checking. The two levelers should 

 tnako occasional comparisons in the same way. All directions should be compared 

 with the magnetic needle as the work progresses, although in a hilly country, and espe- 

 cially where beds of iron ore exist, the check is only a rough one, as the needle is easily 

 influenced by local attraction. Small isolated hills in a level country will also deflect 

 it. As in a chain survey each party must be provided with a topographer. The average 

 error of closure should not much exceed i in 600. 



The hydrographic party also has a transit-man, who works from points on the 

 shore from J to i mile apart, and connected by the other transit-men with the general 

 survey. His duty is to locate the position of the soundings by angles and stadia-dis- 

 tances, a rod being held in the skiff for that purpose. The recorder in the skiff chooses 

 his ranges by eye, keeping as near as practicable in straight lines, and both transit-man 

 and recorder keep tally on the soundings by a time-check, supplemented by special 

 signals at every tenth or twentieth one, as may be desired. 



On large rivers the rate of progress of the parties may average i to ij miles a day, 

 while on small rivers the rate may reach as high as 6 miles. 



The accuracy of stadia surveys depends largely on the clearness of the atmosphere. 

 In the U. S. Lake Surveys the average length of sight was 800 to 1000 feet, with a 

 maximum of 2000 feet. The lines averaged i J miles in length, and a limit of error in 

 closure was allowed of i in 300. 



(4) Triangulation furnishes the most complete system of surveying, because by 

 it latitudes and longitudes and the location of all salient features may be determined 

 with precision. It is applicable to rivers of great width where other methods would not 

 be satisfactory or possible, as well as to those of smaller size. Briefly described, it con- 

 sists in dividing the country to be surveyed into a series of large triangles, one side of 

 one of them being exactly measured for a base-line, and all the other sides and angles 

 being found by triangulation. One side of the last triangle is also measured, thus sen-- 

 ing as a check on the whole work. These large or primary triangles are subdivided into 

 smaller or secondary triangles, and these again into tertiary triangles, thus embracing 

 the whole district. The details are filled in by topography. In this system no angle 

 should be less than 30, as a flat intersection is conducive to error. In the survey of 

 the Mississippi River * the maximum closing error allowed in the triangles was 6 seconds. 

 This required the greatest care in the observations, as an error of one-third of an inch 

 in centering a transit will make a difference of one second in the angle in a distance of 

 i mile. The base-lines in this sun-ey were placed about 75 miles apart, and were meas- 

 ured two or three times, the allowed discrepancy being i in 250,000. A 3oo-foot steel 

 tape was used, supported every 30 feet by hooks fastened to stakes, and stretched to 



* Annual Report, Chief of Engineers, U. S. A., 1891. 



