Instruments Used in Plane Table Survey
Plane table surveying is a graphical method in which field observations and plotting are done simultaneously. To perform this method accurately, a number of instruments and accessories are required. In ITI Draughtsman Civil, understanding these instruments is essential for proper fieldwork and preparation of accurate maps.
The instruments used in plane table survey are simple in design but play a very important role in ensuring precision and efficiency. Each instrument has a specific function in the process of surveying.
1. Plane Table
The plane table is the main instrument used in this method. It consists of a flat wooden board mounted on a tripod stand.
- Usually rectangular or square in shape.
- Made of well-seasoned wood.
- Used to fix the drawing sheet for plotting.
- Can be rotated horizontally.
The plane table provides a stable surface for drawing and must be properly leveled during use.
2. Tripod Stand
The tripod stand is used to support the plane table at a convenient working height.
- Made of wood or metal.
- Has three adjustable legs.
- Provides stability on uneven ground.
3. Alidade
The alidade is a straight-edge ruler with sighting arrangements used to draw lines on the sheet and sight objects in the field.
Types of Alidade
- Plain Alidade: Simple ruler with sight vanes.
- Telescopic Alidade: Provided with a telescope for greater accuracy.
It is used to align the direction of objects and draw rays on the drawing sheet.
4. Plumbing Fork (U-Fork) and Plumb Bob
The plumbing fork is used along with a plumb bob for centering the plane table over the survey station.
- Ensures the plotted point lies exactly above the ground station.
- Helps in accurate positioning of the table.
5. Spirit Level
The spirit level is used to make the plane table horizontal.
- Contains a bubble in a liquid-filled tube.
- Ensures accurate leveling of the table.
6. Trough Compass
The trough compass is used for orientation of the plane table.
- Indicates the magnetic north direction.
- Helps in aligning the table correctly with previous stations.
7. Drawing Sheet
A drawing sheet is fixed on the plane table for plotting the survey details.
- Made of good quality paper.
- Properly stretched and fixed on the board.
8. Accessories
In addition to the main instruments, several accessories are used:
- Pencils: For drawing lines and marking points.
- Eraser: For correcting mistakes.
- Scale: For measuring distances on the drawing.
- Clips or Pins: For fixing the drawing sheet.
9. Importance of Instruments
Each instrument used in plane table survey has a specific role. Proper use of these instruments ensures:
- Accurate plotting of survey data.
- Reduction of errors.
- Efficient fieldwork.
10. Care and Maintenance
- Keep instruments clean and dry.
- Handle alidade and compass carefully.
- Store equipment in a safe place.
- Check leveling and alignment regularly.
Role of Draughtsman Civil
A Draughtsman Civil uses the data obtained through plane table survey to prepare detailed drawings and maps. Proper knowledge of instruments helps in accurate representation of field observations.
Conclusion
The instruments used in plane table survey are simple yet essential for accurate fieldwork. Proper understanding and handling of these tools ensure precise measurement and plotting.
For ITI Draughtsman Civil students, mastering these instruments is important for developing practical surveying skills and producing accurate construction drawings.