THE PLANE TABLE 



ROM a mathematical standpoint the plane table cannot 

 be regarded as an accurate instrument, for there are 

 both theoretical and practical errors in the method 

 of using it which offend the nicely trained instincts of 

 the field engineer; but the accuracy of any map is 

 limited, not so much by that of the field instruments 

 as by the protractor and the proportions of the scale. 

 The fact that a point can be oriented by an alidade 

 as accurately as it can be located on a small scale 

 map has given rise to the growing conviction that for certain classes 

 of topography in open country the plane table and stadia method 

 is not equalled by any other system. 



The scheme of plotting the map in the field and of sketching 

 in all details, as the work progresses, without elaborate mental or 

 written notes and with fewer computations or located points, probably 

 more than offsets, in time and expense, the extra field work and the 

 weather hazard. 



In the hands of a skillful operator the plane table is not ex- 

 celled by any instrument for the graphic control of horizontal 

 position by polar or rectilinear coordinates, and is the only in- 

 strument capable of a rapid solution of the 3-point problem in 

 the field. With it, cumulative errors are overcome, for points 

 may be located without reference to other portions of the survey. 

 Field work may be conducted by the methods of Radiation, 

 Intersection, Resection, Traversing, Radio-Progression, or by the 

 Two- or Three-Point Problems, t 



Traverse methods consist essentially of leveling and orienting 

 the table, drawing lines in the direction sighted and scaling off dis- 

 tances by which each station becomes the origin of a separate polar 

 coordinate system. Intersection methods, however, are generally 

 more accurate and can be more reliably checked, precisely as 

 triangulation in geodetic control is more accurate than instrumental 

 traverse. 



Rules for Graphic Control. * 



1. Occupy the known point first. 



2. Measure base lines twice in opposite directions. The base line 



plotted on the plane table to field scale should not be less than 



two inches in length. 



X No location is good unless made by three intersections. 

 4. Intersection angles at important stations should not be less than 



30. Two lines intersecting produce a minimum error when 



they meet at 90; three lines at 60 or 120 etc. 



* f). M . t/iffzins in Economic Geolofy, Dec. 1913. pp. 729-751. 

 \ See also D. L. Rtaburn in Enjf. News, Mar. 26 % 1914 



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