Plane table surveying also called plane tabling, is a method of surveying in which field work and office work are done simultaneously on a plane table. The field observation are taken and recorded side by side on the sheet fixed upon the plane table and a map of the area is obtained. The use of field book is altogether eliminated. It is most suitable for the survey of details between stations which have already been fixed by theodolite or any other accurate method of surveying.
It is commonly employed for small and medium scale mapping of comparatively large areas where great accuracy is not the main consideration such as for topographical surveys.
Equipment’s are used in Plane Table Surveying
The equipment needed in plane table surveying is detailed with under different heads as follows:
1. The Drawing Board and Tripod: The drawing board which the field measurements are plotted is fixed on the plane table or drawing board. The board is made of well seasoned good quality wood such as teak or pine and is available in sizes. (i) small size: 50 cm, multiply 40 cm, multiply 1.5 cm. (ii) medium size: 50 cm, multiply 50 cm. (iii) large size: 70 cm, multiply 60 cm, multiply 1.5 cm. The upper surface of the board must be perfectly plane. It is mounted on a tripod in such a way that it can be leveled, rotated about a vertical axis and clamped in any position. The tripod is generally of the open frame type and can be folded for convenience of transportation. The board is sometimes fitted with a leveling head or a ball and socket arrangement for the purpose of leveling the table.
2. The Alidade: The alidade is a straight rule fitted with such an arrangement as to provide a line of sight. It is used for sighting objects and drawing rays along one of its beveled edges known as ruling or working or fiducial edge is graduated so as to serve as a scale for plotting distances. It is available in two forms (a) Plain alidade (b) Telescope alidade.
(a) Plain Alidade: It consists of a wooden or brass ruler 50 to 75 cm long, or of a length equal to the smaller dimension of the broad and is provided with a pair of vertical sight vanes at its ends. One of the vanes is provided with a narrow slit, while the other is open and carries a horse-hari or fine-wire. A small circular hubble is sometimes attached to the alidade for leveling the table. The alidade with plane vanes provides a sufficiently definite line sight for many purposes, but is some what inconvenient for work in hilly country owing to the limited range of inclination possible in the line of sight. The highly inclined sights can be taken by stretching a thread tightly between the centers of the tops of the vanes and sighting the object through the eye-slit and this thread.
(b) Telescope Alidade: In this type of alidade, a telescope is fitted on the straight rule which increases considerably the accuracy and the range of sighting. The telescope is provided with a vertical circle to measure the angles in the vertical plane and stadia-wires to measure horizontal distances. Thus the telescopic alidade serves the purpose of a small theodolite. It is commonly used when the sights are long and much inclined.
3. Accessories: The accessories to the plane table are:
(a) A Tough Compass: It is used for marking the direction of the magnetic meridian on the sheet of the plane table.
(b) A Plumbing Fork or U-Frame: With a plumb-bob is used for centering the table.
(c) A Spirit Level: (If not to the alidade) for leveling the plane table.
Besides these the other accessories are drawing sheet, a water proof cover to protect the drawing sheet and drawing instruments.
4. Drawing Sheets: For other than rough work, the paper used for plane tabling should be of the best quality. Paper is very sensitive and liable to expand or contract due to changes in the humidity of the atmosphere. To lessen this effect, it should be well seasoned for about a week by exposing it alternately to a damp and dry atmosphere. This effect is practically negligible if double mounted sheets are used for the plotting work. For in damp climates, sheet of zinc and of celluloid may be used, and to reduce strain on the eyes, the paper may be faint green or grey.
The paper may be fixed on the drawing board by means of drawing pins, spring clips, or screw-clamps. But the better way is to paste down the edges of the paper.
The sheet should never be-folded or rolled but should be carried flat in a suitable cover.