What are the instruments used in plane table surveying?

Short Answer:

The instruments used in plane table surveying include a plane table, tripod, alidade, spirit level, plumb bob or plumbing fork, and drawing accessories. These tools help the surveyor to observe and plot the area directly on-site.

The plane table provides a flat surface for drawing, and the alidade is used to sight and draw lines. The tripod holds the table, while the spirit level helps in leveling it. A plumb bob is used to center the table, and pencils, scale, and clips are used for recording the details.

Detailed Explanation:

Instruments used in plane table surveying

Plane table surveying is a quick and practical method of surveying where observations and map-making are done directly in the field. It requires a simple set of instruments that allow the surveyor to measure angles, draw directions, and record features on a drawing sheet in real-time. Each instrument has a specific role in setting up the table, orienting it, sighting objects, and preparing the map.

Here are the main instruments used:

1. Plane Table

The plane table is the most important instrument. It is a smooth, flat wooden or metal board fixed on a tripod. The drawing sheet is attached to this board. It serves as the surface where observations are directly plotted. The board must be rigid and lightweight for ease of use in the field.

2. Tripod

The tripod is a three-legged stand that holds the plane table steady. It allows height adjustment and supports the table in a level position. The tripod should be strong, stable, and easy to set up on uneven ground.

3. Alidade

The alidade is a sighting device with either plain or telescopic vanes. It is placed on the drawing sheet over the plane table to sight points on the ground and draw lines (called rays) in the direction of those points.

  • Plain alidade has a metal or wooden ruler with two vertical sight vanes.
  • Telescopic alidade includes a small telescope for longer or more accurate sighting.

4. Spirit Level

The spirit level is a small device with a bubble tube, used to level the plane table. A level surface is essential for accuracy in observations and drawing. The bubble in the tube must stay at the center to ensure the table is horizontal.

5. Plumbing Fork with Plumb Bob

The plumbing fork is a U-shaped metal tool used with a plumb bob to center the plane table exactly over a survey station mark on the ground. The plumb bob hangs vertically through the fork, showing the point on the ground below the center of the table.

6. Drawing Accessories

These include pencils, rulers, drawing sheets, erasers, clips or thumbtacks, and sometimes a protractor. They are used for sketching, scaling, and marking survey data directly on the sheet.

  • Drawing sheet is the surface where all observations are recorded.
  • Clips or tacks help to keep the sheet firmly attached to the board.

Optional Instruments:

  • Compass: Used for orientation when the direction of north is required.
  • Cover or umbrella: Used to protect the drawing from wind, sun, or rain.

All instruments should be handled with care and checked for accuracy before use. Good maintenance ensures consistent performance and reliable survey results.

Conclusion

The instruments used in plane table surveying include the plane table, tripod, alidade, spirit level, plumb bob, and drawing tools. Each plays a specific role in helping the surveyor observe and record ground features directly in the field. Their proper use ensures accurate, quick, and simple survey work.