- Methods of Plane Table Survey: Three methods – Radiation, Intersection and Traverse.
- Radiation Method: Rays are drawn from one station to all points; suitable for small areas.
- Intersection Method: Rays are drawn from two stations; used for large areas and distant objects.
- Traverse Method: Survey is done by moving from one station to another along base lines.
- Types of Traverse:
Open Traverse – does not return to starting point.
Closed Traverse – ends at starting point forming a closed figure. - Advantages: Map prepared in field, quick method, less chance of missing details.
- Disadvantages: Not suitable in rainy weather, heavy instrument, difficult for very large areas.
Key Points
Key Points: Methods of Plane Table Survey
Key Points: Plane Table Survey
- Plane Table Survey: Field work and plotting are done together on the plane table.
- Main Instruments: Plane table, alidade, spirit level, plumb fork, trough compass, chain, measuring tape, arrows and ranging rods.
- Alidade: Used for sighting objects and drawing rays on the map.
- Levelling & Positioning: Spirit level levels the table; plumb fork fixes the exact ground station.
- Trough Compass: Used to find magnetic north direction.
- Chains: Three types – Engineer’s chain (100 ft), Gunter’s chain (66 ft), and Metric chain (10, 20, 30 m).
- Other Measuring Tools: Measuring tape measures distances; arrows mark chain lengths; ranging rods help in alignment and marking survey limits.
Key Points: Surveying
- Meaning of Surveying: Surveying is the process of measuring and determining the positions of points on the earth’s surface.
- Importance of Surveying: It helps in preparing maps, layout plans, boundaries and development projects.
- Types based on Accuracy:
Triangulation – very accurate method.
Traversing – quicker but less accurate. - Types based on Purpose: Topographical, Cadastral, Geological and Military surveys.
- Types based on Instruments: Chain and Tape Survey, Plane Table Survey and Prismatic Compass Survey.
Key Points: Precautions for Conducting Plane Table Survey
- Precautions: Ensure all instruments are in working condition, clamp the table tightly, fix drawing sheet properly and choose suitable scale.
- Levelling, Centering & Orientation:
Levelling – Make the table horizontal using spirit level.
Centering – Place table exactly over ground station using plumb bob.
Orientation – Set table in correct direction. - Fixing Base Line: Select a suitable base line which is central, level and clearly visible.
- Drawing of Rays: Use alidade to sight objects and draw rays from the base line.
- Re-orientation: Done at new stations by back sighting or by using trough compass.
