Plane Table Survey
Plane table surveying is a graphical method of surveying in which the field observations and plotting are done simultaneously. It is one of the simplest and most effective methods used in surveying, especially for small and medium areas. In ITI Draughtsman Civil, the study of plane table survey helps students understand practical field techniques and direct mapping methods.
1. Introduction to Plane Table Survey
Plane table surveying is a method in which the plan of the area is drawn directly in the field on a drawing sheet fixed to a plane table. Unlike other methods where measurements are taken first and plotting is done later, in plane table survey both operations are carried out at the same time.
This method is particularly useful for preparing topographical maps and filling in details quickly and accurately. It is widely used for surveys where high precision is not required.
2. Principles of Plane Table Survey
Plane table surveying is based on the principle of parallelism. This means that the lines drawn on the paper are parallel to the corresponding lines on the ground.
- The direction of the line on the paper is kept parallel to the direction on the ground.
- The position of points is fixed by drawing rays from known points.
3. Instruments Used in Plane Table Survey
- Plane Table: A flat wooden board mounted on a tripod.
- Alidade: A straight edge used for sighting and drawing lines.
- Plumbing Fork: Used for centering the table over a station.
- Spirit Level: Used for leveling the table.
- Tripod Stand: Supports the plane table.
- Drawing Sheet: Fixed on the table for plotting.
4. Setting Up of Plane Table
Proper setup of the plane table is necessary for accurate work. The following steps are involved:
- Fixing: The table is mounted on a tripod stand.
- Leveling: The table is made horizontal using a spirit level.
- Centering: The table is placed exactly over the survey station using a plumbing fork.
- Orientation: The table is rotated so that it is parallel to its previous position.
5. Methods of Plane Table Survey
(a) Radiation Method
- Used when all points are visible from one station.
- Lines are drawn from the central station to different points.
(b) Intersection Method
- Used when points are far away or inaccessible.
- Points are located by intersection of rays from two stations.
(c) Traversing Method
- Used for large areas.
- Series of connected stations are used.
(d) Resection Method
- Used to locate the position of the table.
- Based on known points already plotted.
6. Advantages of Plane Table Survey
- Fieldwork and plotting are done simultaneously.
- Reduces chances of errors.
- No need for complex calculations.
- Suitable for quick surveys.
- Useful for filling in details.
7. Disadvantages of Plane Table Survey
- Less accurate compared to modern methods.
- Not suitable for large areas.
- Cannot be used effectively in rainy or windy conditions.
- Requires clear visibility of points.
8. Applications of Plane Table Survey
- Preparation of topographical maps.
- Survey of small areas and sites.
- Filling in details between control points.
- Preliminary surveys for engineering projects.
9. Errors in Plane Table Survey
- Errors in centering and leveling.
- Improper orientation of table.
- Inaccurate sighting with alidade.
- Movement of table during work.
10. Precautions
- Ensure proper leveling and centering.
- Keep the table stable during work.
- Check orientation frequently.
- Use sharp pencil for accurate plotting.
Role of Draughtsman Civil
A Draughtsman Civil uses plane table survey data to prepare site plans and layouts. Since plotting is done in the field itself, it helps in quick decision-making and accurate representation of features.
Conclusion
Plane table surveying is a simple and efficient method of surveying where fieldwork and plotting are done together. It is suitable for small areas and quick surveys.
For ITI Draughtsman Civil students, understanding plane table survey is important for developing practical surveying skills and preparing accurate maps and drawings.