Plane Table Survey

• Plane tabling is a graphical method of survey in which the field observations and plotting proceed simultaneously.

Instruments used:

 Plane table: Three types of the plane tables are commonly used in practice:

1. The traverse tables: Leveled by adjusting tripod less, usually by eye estimation.
2. Johnson table: Consists of drawing bound usually 45 X 60cm or 45 X 75cm. head consists of ball and socket joint.
3. Coast survey table: Leveling done very accurately with the help of three foot screws.

 Alidade: A straight edge with same form of fighting device. Two types: .

1. Plain alidade:  Consists of metal of wooden rule with two vanes at the ends. The working edge against which lines are drawn is also known as “bevelled edge or edge”.         Not very much suitable on hilly area since inclination of the line of sight is limited.
2. Telescopic alidade:

Significance: We can take inclined sights.

• Accuracy and range of sights are increased by its uses.
• Horizontal distance between the instrument and the point sighted can be computed by taking stadia readings on the staff kept at the point.
•  Elevation of points can also be computed by using usual technometric relations.
• All the above things can be done very quickly

Plumbing fork: used in large scale work. Meant for centering the table are the point of station occupied by the plane table when the plotted position of point is already known as the sheet.

Spirit level : For proper leveling

Compass : For orientation - generally tough compass is used.

Drawing paper : of superior quality.

        

• Working operations:

Fixing

Setting 

Consist of :

1. Leveling the table
2. Centering 
3. Orientation: The process of putting the plane table into some fixed direction so that line representing a certain direction on the plane is parallel to that direction on the ground.

• If orientation is not done, the false will not be parallel to itself at different positions resulting in an overall distortion of map.
• Significance: This is an essential condition to be fulfilled when more than one instrument station is to be fixed.
• Methods of orientation: 

 Orientation by through compass : used when

• speed is more important than accuracy
•  there is no second point available for orientation. .

 Orientation by back sighting : Two methods

1. when it is possible to set the plane table on the point already plotted on the sheet by way of observation from previous table. When it is not possible to set the plane table on the point already plotted. The method is also known as “Resection".

• Methods of plane tabling:

1. Radiation 
2. Intersection
3. Traversing 
4. Resection .

The First two methods arc generally employed for locating the details while the other two methods are used for locating the plane table stations

 Radiation: Suitable when the distances are small (with in a tape length) and are single instrument can control  the points to be  detailed. 

Intersection: 

• Also known as graphic triangulation.
• Suitable for hilly areas

Traversing: used to plot areas.

Resection: The process of determining the plotted position of the station occupied by the plane table, by means of sights taken towards known points, location of which have been plotted. Resection can be done by the following methods. 

•  By compass
• By back sighting
• Three point problem
• Two point problem 

Three point problem: The location of plane table station can be located by three well defined points on ground.

1. Mechanical (Tracing paper) method
2. Graphical (Bessel’s) method
3. Trial and error (Lehmann’s) method

Two point problem: Plane table station can be located by two well defined points on the ground.