Plane Table Surveying

Plane table surveying is a fast and efficient surveying method that allows for simultaneous field observations and plotting of plans. This technique is particularly useful for capturing the geometrical conditions of a site directly onto a map using a plane table and an alidade. The details of topography can then be organized and finalized on the map, making it a versatile tool in the field of surveying.

Equipment Used in Plane Table Surveying

A range of equipment is necessary to conduct a plane table survey effectively. The main tools include:

• Plane Table: The central tool for plotting
• Alidade: Used for sighting, can be telescopic or simple
• Plumb Bob and Plumb Fork: For centering the table
• Compass: Assists in orientation
• Spirit Level: Ensures the table is leveled
• Chain and Ranging Rods: For measuring and marking distances
• Tripod: Holds the plane table steady
• Drawing Sheet and Tools: For creating the map
• Paper Clips or Screws: To secure the drawing sheet

Setting Up the Plane Table

To achieve accurate results, the plane table must be set up carefully following these steps:

1. Fixing the Plane Table: Secure the table to the tripod and attach the drawing sheet using paper clips or screws. The sheet must remain in a fixed position throughout the survey.
2. Leveling the Plane Table: Use a spirit level to ensure the table is perfectly horizontal. For small-scale surveys, leveling by eye estimation may suffice.
3. Centering the Plane Table: Center the table using a plumbing fork so that plotted points align exactly with their corresponding ground points.
4. Orientation of the Plane Table: Rotate the table to ensure that the plotted lines on the drawing sheet are parallel to the corresponding lines on the ground. Use a compass or back sighting for this purpose when multiple stations are involved.

Methods of Plane Table Surveying

Plane table surveying employs four primary methods, each suited to specific requirements:

1. Radiation: Sight and plot points from a single location.
2. Intersection: Determine a point by intersecting rays from two known stations.
3. Traversing: Connect a series of straight lines by moving the table from one point to another.
4. Resection: Locate the unknown position of the plane table by sighting known points.

Detailed Methods of Plane Table Surveying

1. Radiation Method

In the radiation method, the plane table is placed at a single location (e.g., point O). From this point, various stations (A, B, C, etc.) are sighted using the alidade. The corresponding points (a, b, c, etc.) are plotted on the drawing sheet to scale, creating a direct representation of the site.

2. Intersection Method

This method involves plotting a point by intersecting two rays drawn from two known stations. For example, place the equipment at station P and sight stations A, B, and Q. Plot lines on the drawing sheet from station P. Then move to station Q, sight stations A, B, and P, and draw additional lines. The intersection of the lines from stations A and B gives the desired location.

3. Traversing Method

Traversing involves a series of straight lines connecting multiple points. Start by placing the table at one point (e.g., A) and sight towards the next point (B). Measure the distance AB and plot it on the drawing sheet. Repeat the process from B to C and so on, checking accuracy at intervals. The result is a traversed route plotted on the map.

4. Resection Method

Resection is used when the plane table’s location is unknown. By sighting known points, the table’s position can be determined. This method is further divided into:

• The Three-Point Problem: Solve for a location by sighting three known points.
• The Two-Point Problem: Solve for a location by sighting two known points.

The Three-Point Problem

In the three-point problem, the table is placed at a point where three known points are visible. The location is determined using one of the following methods:

1. Tracing Method: Use tracing paper to plot rays from the three points and align them on the map to mark the position.
2. Lehmann Method: Reduce errors by shifting the table and plotting multiple triangles of error until they converge.
3. Analytical Methods: Use measured angles and distances to calculate the position using formulas.
4. Graphical Method: Represent angles and lengths on a graph to determine the location.

The Two-Point Problem

The two-point problem involves sighting two known points to determine the table’s location. This can be further divided into:

• Case 1: When the table can occupy the controlling stations.
• Case 2: When the table cannot occupy the controlling stations and an auxiliary point is used to assist in orientation.

Conclusion

Plane table surveying is a versatile and efficient method that combines field observations and map plotting. Its four methods—radiation, intersection, traversing, and resection—provide flexibility for various surveying needs. Despite its simplicity, it requires careful setup and orientation to achieve accurate results. Plane table surveying remains a valuable tool for topographical and construction surveys, offering a practical solution for creating detailed maps directly in the field.