CHAPTER IV. 

 TOPOGRAPHICAL SURVEYS AND LEVELING. 



< 



TOPOGRAPHICAL SURVEYS. 



A TOPOGRAPHICAL survey of a river is made for the purpose of locating its course 

 geographically, and to establish such points as may be required in the prosecution of 

 the works of improvement. The survey may be made with (i) the transit and chain, or 

 tape, and level, as is customary in railroad work, or (2) with plane-table and stadia- 

 rods, or (3) with transit and stadia-rods, or (4) by triangulation. 



(i) The methods of Transit and Chain surveys are so well known as to call for but 

 little explanation. If the stage of the river is sufficiently low, it may be possible to 

 make the survey in the bed, and this will greatly simplify matters, because an approxi- 

 mately level surface will be secured for the measurements, without obstructions from 

 brush, buildings, timber, etc. When this condition does not exist, it will be necessary 

 to run a line along one or both banks. Stakes should be set every hundred feet, and 

 from these offsets may be made to determine the shore lines and other points desired, 

 such as high- and low-water marks, rocks, bars, etc. The angles are of course meas- 

 ured with the transit, by which the stakes are also lined in, and the observations should 

 be approximately checked with the magnetic needle. The level party follows the transit, 

 taking levels to the water surface at sufficiently close intervals to determine the slope, 

 and establishing bench-marks at points from half a mile to a mile apart, or checking 

 upon those of a former survey, if any has been made. The topographer closely follows 

 the level party with a hand-level and note-book, recording the general cross-section 

 of the territory passed over, either pacing his distances, or, where great accuracy is 

 desired, using a tape-line. He should also make sketches, more or less complete, of the 

 general features of the line, as these are often of much value in plotting the survey. His 

 notes also give the contours of the ground at certain intervals, say 5 feet.* 



A transverse profile or section of the river-bed should be made at or near each 

 bench-mark, and at the probable locations of works of improvement such sections should 

 be taken from 100 to 400 feet apart, by one or other of the methods described in the 

 next chapter. Notes as to the character of the bottom and of the banks should also 

 be made, so that an approximate geological section can be constructed. 



* A more complete description of the methods of topographical and hydrographical surveying will be 

 found in "The Theory and Practice of Surveying," by Professor J. B. Johnson. 



20 



