A plane table survey is a graphical method of survey in which the field observation and plotting are done simultaneously.
This type of survey is simple and cheapest than the theodolite survey. It is most suitable for small scale maps because this survey becoming hard to plot for large scale plots and maintain accuracy is difficult.
The plan is drawn by the surveyor in the field while the area to be surveyed is before his eyes. Therefore there is no possibility of omitting the necessary measurements.
Equipment And Accessories For Plane Tabling
The following instruments are used in plane table surveying
- Equipment
- Plane table
- Tripod
- Alidade
- Accessories
- Trough compass
- Spirit level
- U fork with plumb bob
- Waterproof cover
- Drawing paper
- Pins
- Drawing Accessories
A) Equipment for Plane Table Survey
The following equipment is used in plane table surveying
Plane Table
The Drawing board for plane tabling is made from well-seasoned wood with its upper surface exactly plane.
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It is normally rectangular in shape with a size of 75 cm x 60 cm
It is mounted on a tripod and clamps are provided to fix it in direction. The table can be avoided about its vertical axis and be clamped in any position, when necessary
Three types of plane tables are common in use
- Simple plane table
- Johnson plane table
- Coast survey plane table
Tripod
The plane table is mounted on a tripod
The tripod is generally of open frame type, combining rigidity lightness.
The tripod may be made to fold for convenient transportation
Tripod is provided with three-foot screws at its top for levelling the plane table
Alidade
The alidade is useful for establishing a line of sight
Two types of alidade are used
- Simple alidade
- Telescopic alidade
I) Simple Alidade
It is used for ordinary work
It generally consists of gunmetal or wooden rule with two vertical vanes at the ends
The eye vane (sight vane) is provided with a narrow slit while the object vane is open and carries horsehair. Both the slits, thus provide a definite line of sight that can be made to pass through the object to be sighted
To draw the rays, one of the edges of the alidade is bevelled and this perfectly smooth working edge is known as the fiducial edge. The fiducial edge is graduated to facilitate the plotting of distances to a scale
II) Telescopes Alidade
The telescope alidade is used when it is required to take inclined sights
It essentially consists of a small telescope with a level tube and graduates are mounted on the horizontal axis
It gives higher accuracy and more range of sights.
B) Accessories for Plane Table Survey
Trough Compass
The through compass is required for Drawing the line showing magnetic meridian on the paper. It is used to orient the table to the magnetic meridian
When the freely suspended needle shows a 0° – 0° reading at each end, a line is drawn on the drawing paper which represents the magnetic north
Spirit Level
A spirit level is used for ascertaining if the table is properly level
The table is levelled by placing the level on the board in two positions at right angles and getting the bubble central in both positions
Also at the beginning of the work, it is used for transferring the ground points on the sheet
U – Fork with Plums Bob
U Fork with plumb bob is used for centring the table over the point or station occupied by the plane table when the plotted positions of that point are already known on the sheet
Water Proof Cover
An umbrella is used to protect the drawing paper from rain
Drawing Paper
Drawing paper is used for plotting the ground details
Advantages of Plane Table Surveying
- The plan is drawn by the surveyor himself while the area to be surveyed is before his eyes. Therefore there is no possibility of omitting the necessary measurements.
- The surveyors can compare the plotted work with the actual features of the area
- It is simple and cheaper than the theodolite survey
- It is most suitable for small scale maps
- No great skill is required to produce a satisfactory map and the work may be entrusted to a subordinate
- It is used in magnetic areas where the compass may not be used
- The mistakes in writing field books are eliminated
- It is the most rapid method of surveying
- Inaccessible points can be easily located by the intersection
Disadvantages or Limitations of Plane Table Surveying
- It is not intended for very accurate work
- It is not suitable in monsoon
- It is essentially a tropical instrument
- Due to heaviness, it is inconvenient to transport
- Since there are so many Accessories there is the likelihood of these be lost
