TOPOGRAPHICAL SURVEYS AND LEVELING. 23 



a uniform tension by a weight of 16 pounds, with a thermometer every 100 feet. The 

 station distances were marked on strips of zinc fastened to the tops of the stakes. The 

 position of each -base was checked by astronomical observation. In closing on trian- 

 gulation points the error allowed did not exceed 5 minutes for the angles and i in 500 

 for distance. Contours in the bottoms were taken 5 feet apart, and 20 feet apart on 

 the bluffs, and stadia sights were limited to 500 meters. 



Permanent survey marks were placed about every 3 miles, two to a set, and on 

 each side of the river on a line normal to the stream, one being placed near the bank, 

 the other half a mile back. They were made of iron pipes set on a flat tile, and with 

 the tops projecting about a foot above the ground. 



LEVELING. 



Levels may be divided into Ordinary and Precise. 



Ordinary levels are run with a regular wye-level, and the degree of accuracy attained 

 depends principally upon the care with which the work is done. Good leveling, what- 

 ever be its kind, requires a steady atmosphere, with sights of equal length, not over 

 100 meters under favorable conditions, and reduced to one-third of that when the atmos- 

 phere is unsteady. The level should always be kept in the shade as far as possible, and 

 should be tested daily for adjustment. Both the level and the rod must be solidly 

 set, and care taken always to see that the bubble is in the proper position at time of 

 reading. It is also necessary that the level-rod be held exactly plumb. 



In the level work of the U. S. Geological Survey the sights were limited to 300 feet, 

 and were made equal as nearly as possible, the distances being obtained by pacing. 

 Work was suspended during high winds or when the atmosphere was "boiling" on a 

 hot day. The rodmen, who were required to use plumbing-levels on the rods, were 

 supplied with conical steel pegs, 6 to 12 inches long, which were driven into the ground 

 for turning-points, while for bench-marks copper or bronze bolts, set in rock or iron 

 posts, were used. The error of altitude was limited to o.osv/distance in miles. 



On the Mississippi River survey, where a line of levels was run along each bank, 

 the parties were required to check on each other every 3 miles, and the error of closure 

 was limited to 0.2 foot for ordinary lines, and to 0.05 foot for the bench-marks, which 

 were placed about a mile apart. 



Precise levels are made with an instrument constructed for the especial purpose. 

 Their object is to determine with accuracy the. elevations, with reference to sea-level, 

 of inland points too distant for reliance upon ordinary methods of leveling. , Most coun- 

 tries have therefore adopted the system of precise leveling for ascertaining the elevation 

 required, and the United States has been using it for a number of years in its more 

 complete surveys, establishing permanent bench-marks at short intervals. 



The accuracy with which this class of work is done is not all due to the instrument 

 used, but much of it must be credited to the care taken and the methods employed. 



