Plane table
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A plane table is a device used in surveying and related disciplines to provide a solid and level surface on which to make field drawings, charts and maps.
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[edit] History
The first known instrument for directly producing a drawing of a site was developed by Johann Richter, also known as Johannes Praetorius[1], a Nuremberg mathematician, in 1610.[2][dubious ] This circular table, called a tabula praetoriana, mensula praetoriana or Pretorian table, used a simple alidade and allowed a piece of paper to be slipped under the alidade for drawing.
Later devices adopted a rectangular table and enhanced the type and features of the alidade.
[edit] Plane table construction
This shows a plane table with part of the surface of the table cut away to show the mounting on the tripod. The mount allows the table to be levelled. On the table, the alidade with telescopic sight is seen
A plane table consists of a smooth table surface mounted on a sturdy base. The connection between the table top and the base permits one to level precisely the table, using bubble levels, in a horizontal plane. The base, often a tripod, is designed to support the table over a specific point on land. By adjusting the length of the legs, one can bring the table to being approximately level regardless of the roughness of the terrain.
[edit] Use of a plane table
In use, a plane table is set over a point and brought to precise horizontal level. A drawing sheet is attached to the surface and an alidade is used to sight objects of interest. The alidade, in modern examples of the instrument a rule with a telescopic sight, can then be used to construct a line on the drawing that is in the direction of the object of interest.
By using the alidade as a surveying level, information on the topography of the site can be directly recorded on the drawing as elevations. Distances to the objects can be measured directly or by the use of stadia marks in the telescope of the alidade.
[edit] References
- ^ Vai, Gian Battista, Caldwell, W. G. E. The Origins of Geology in Italy Google books online copy
- ^ Laussedat, Amié, Recherches sur les instruments, les méthodes et le dessin topographiques, Paris, 1898-1902, two volumes
- Raymond Davis, Francis Foote, Joe Kelly, Surveying, Theory and Practice, McGraw-Hill Book Company, 1966 LC 64-66263
[edit] External links
- Telescopic Plane Table Alidade in the collection of the Smithsonian National Museum of American History. This image shows the bubble levels on the base.
