Short Answer:

Plane table surveying is a method of field surveying where observations and plotting are done directly on the drawing board mounted on a tripod. In this method, the map or layout is drawn at the site itself using a plane table, alidade, and other tools. It is simple, quick, and useful for small-scale surveys.

This type of surveying is especially helpful in areas where the land is not too large or complex. It avoids the need for separate field notes and reduces errors because the drawing is made while observing the ground features directly.

Detailed Explanation”

Plane table surveying

Plane table surveying is a practical method used in civil engineering and land surveying to prepare maps and plans directly in the field. It involves the use of a plane table, which is a flat, leveled board mounted on a tripod. With this method, both observation and plotting happen at the same time, which makes the process fast and efficient for small and medium-sized areas.

Unlike other surveying methods where data is collected in the field and plotted later in the office, plane table surveying allows the surveyor to see the terrain and draw it immediately. This minimizes the chances of mistakes due to missing or wrong field notes.

Main instruments used in plane table surveying:

Plane Table: A flat board used as the drawing surface.
Tripod: A stand that supports the plane table and allows leveling.
Alidade: A sighting device used to draw rays and measure directions.
Spirit Level: Used to level the table.
Plumb Bob or Plumbing Fork: Used to center the table over a specific ground point.
Drawing tools: Pencils, paper, clips, and ruler for plotting the map.

Steps in plane table surveying:

Setting up the table: The table is fixed over a point using the tripod, centered and leveled using a plumb bob and spirit level.
Orientation: The table is aligned or oriented in the same direction it had at the previous station using the alidade or a compass.
Plotting the station: The ground station is plotted on the sheet at a fixed scale.
Sighting and drawing: The surveyor sights features or other stations through the alidade and draws rays (lines) from the station point.
Measuring distances: Distances can be measured by tape or estimated using methods like intersection, radiation, or traversing.
Completing the map: The entire map is drawn by repeating the above process at different stations.

Methods used in plane table surveying:

Radiation Method: Used when all points are visible from a single station.
Intersection Method: Used when distances are not directly measured but positions are found by intersecting rays.
Traversing: Used for surveying long narrow areas like roads.
Resection: Used to find the position of the table by sighting known points.

Advantages:

Fast and simple for small areas.
No need for separate field notes.
Errors are noticed and corrected immediately.
Helps in clear understanding of the terrain.

Limitations:

Not suitable for rainy, windy, or rough weather.
Less accurate than theodolite or total station surveys.
Requires flat surface for setting up the table.

Plane table surveying is still used in many rural, educational, or small-scale projects where quick, on-the-spot mapping is needed.

Conclusion

Plane table surveying is a method where both observation and drawing are done on-site using simple tools like a plane table and alidade. It is easy to use, saves time, and is suitable for small, clear areas where high accuracy is not critical.