THE TOPOGRAPHICAL SURVEY OF INDIA. 249 



this result, fixed stations are established in the vicinity 

 of the great trigonometrical series. Where this plan 

 cannot be adopted, a network of large symmetrical 

 triangles is thrown over the district to be surveyed, or 

 boundary series of triangles are carried along the outline 

 of the district or along convenient internal lines. The 

 former of these methods is applicable to a hilly district, 

 the latter to a flat country. 



When the district to be surveyed has been triangu- 

 lated, the work of filling-in the topographical details id 

 commenced. Each triangle being of moderate extent, 

 with sides from three to five miles in length, and the 

 angular points being determined, as we have seen, with 

 great exactness, it is evident that no considerable error 

 can occur in filling-in the details. Hence, methods can 

 be adopted in the final topographical work which would 

 not be suitable for triangulation. The triangles can 

 either be ' measured up,' or the observer may traverse 

 from trigonometrical point to point, taking offsets and 

 intersections ; or, lastly, he may make use of the plane 

 table. The two first methods require little comment ; 

 but the principle of plane-tabling enters so largely into 

 Indian surveying, that this notice would be incomplete 

 without a brief account of this simple and beautiful 

 method. 



The plane-table is a flat board turning on a ver- 

 tical pivot. It bears the chart on which the observer 

 is planning the country. Suppose, now, that two points 

 A and B are determined, and that we require to mark 

 in the position of a third point c : It is clear that if we 



