86 A MANUAL OF TOPOGRAPHIC METHODS. 



gulation points in order to check up accumulated errors. If it were prac- 

 ticable or economic to carry on all the work of location by intersection, this 

 would be the most accurate and on most accounts the best way to effect it, 

 but it is only in limited localities, such as high mountain regions, where bold 

 topographic forms predominate and where there is little or no culture, that 

 the method of intersection is practicable for locating all necessary points. It 

 is probable that in nine-tenths of the area of the United States it will be 

 found necessary to locate the details of topography, culture, and drainage 

 by means of traverse lines. In different parts of the country the relative 

 extent to which the two methods can be applied depends upon various 

 circumstances, .principally the amount of relief of the surface and the prev- 

 alence of forests. Thus upon the Atlantic Plain, which is densely covered 

 with forest, and which is very level, it is necessary to use the traverse 

 method exclusively, including even the primary control. Passing from this 

 as an extreme case, through rolling and hilly country to the high sharp 

 mountains of the West, the triangulation method becomes more and more 

 prominent while the traverse method finally becomes used but little, except 

 in the details of roads and other cultural features. 



For executing traverse work various instruments have been in use for 

 measuring both distances and directions. For direction there have been 

 used theodolites of various forms and prismatic compasses and for distances 

 the stadia and the wheel. 



At present all traverse work is done with plane tables, upon which the 

 directions and distances are platted directly. The plane table used for this 

 purpose is of the simplest possible form, consisting of a board about 15 

 inches square, into one edge of which is set a narrow box containing a com- 

 pass needle 3 inches in length. This table is supported by a tripod of light 

 construction, without leveling apparatus, the leveling of the instrument 

 being effected with sufficient accuracy by the tripod legs. A single screw 

 fastens the board to the tripod head and the adjustment in azimuth is made 

 by simply turning the board with the hand. It is held in place by friction. 

 The table is adjusted in azimuth, or oriented, by means of the compass 

 needle — that is, it is turned until the needle rests opposite the zero marks 

 in the compass box, and is thus always made approximately parallel to 

 itself, provided the magnetic declination remains constant. 



