TOPOGRAPHICAL SURVEYS AND LEVELING. 21 



(2) Plane-table surveying has had an extensive application upon river and lake 

 surveys in the United States. A plane-table is, as ordinarily used, a drawing-board, 

 provided with a movable straight-edge or ".alidade," to which a telescope is attached 

 The table is mounted for convenient use upon a tripod, and is provided with bubbles 

 for leveling and a needle for magnetic bearings. As the rule moves exactly as docs 

 the telescope, it is possible to lay out at once upon the paper fastened upon the board 

 all the directions taken by the telescope to whatever scale is desired. 



To use the plane-table, it is first necessary to establish a base line of known length 

 and to plot it upon the paper to the scale proposed for the map. The instrument is 

 then set on one extremity or station of this line, and the fiducial edge of the ruler 

 is brought into coincidence with its two points, and the table revolved until the opposite 

 station is in the line of sight. By clamping the table and using the slow-motion screw 

 the setting is completed. The directions of any other objects or points may then be 

 plotted by moving the alidade, short lines being drawn in at their supposed distances, 

 and marked in such a manner as will identify them. When all the directions have been 

 taken that are desired, the table is set up over the other station of the base line, from 

 which directions to the same objects are taken and plotted. The intersections of these 

 new lines with those taken from the first station will accurately fix the positions of the 

 objects themselves upon the paper. 



For points of great importance it is well to take observations from still another 

 station, and thus have a check upon the accuracy of the work. 



The topography is added to the sheet as the work progresses, either by the use of 

 stadia-rods or tape or chain measurements. 



The level must of course be used, as in the method first described, for obtaining 

 the profile and elevations, and fixing the bench-marks. 



(3) Stadia surveys are frequently sufficiently accurate for river work, and this 

 method, which is rapid and economical, is applicable to rivers up to one-half mile in 

 width. By its use it is possible to dispense with the chain, and sometimes with the 

 level also, if only approximate results are required. Measurements are obtained with 

 as much accuracy as can be done with a chain in ordinarily broken country. 



In the stadia method the distances are measured by noting what portion of a gradu- 

 ated rod is seen between two crosswires placed in the telescope of the transit for that 

 purpose. With most instruments these wires are so placed that a division of i foot 

 on the rod will exactly coincide with the space between them at a distance of 100 feet 

 from the instrument, plus a certain amount (varying with each instrument, and usually 

 between n and 15 inches), dependent on the focal length of the telescope. For levels 

 as accurate as are required for the location of locks and dams it is necessary to use the 

 level, as in the preceding methods, but for ordinary elevations the vertical circle will 

 give the angle of an object, and the tangent of this angle into the distance measured 

 horizontally will give the distance below or above the instrument. 



Where the river to be surveyed is wide, it will be necessary to have two transit 



