Short Answer:

Radiation and intersection are two important methods used in plane table surveying for locating and plotting points directly in the field. In the radiation method, rays are drawn from a single station to different points, and distances are measured and plotted along these rays. It is useful when all points are visible from one place.

In the intersection method, two known stations are used to draw rays toward an unknown point. The point is fixed where the two rays cross on the drawing sheet. This method is helpful when direct measurement of distances is not possible, like across a river or an obstacle.

Detailed Explanation:

Radiation and intersection in plane table surveying

Plane table surveying allows the surveyor to observe and draw the ground features directly on a map while on site. Among the various methods used, radiation and intersection are two commonly applied techniques depending on visibility and access to points. Both methods help in locating features accurately but are used under different conditions.

Radiation Method

In the radiation method, the plane table is set up at a single known station. From this point, multiple features or objects in the surrounding area are visible. The process involves the following steps:

The drawing sheet is fixed and oriented.
Rays are drawn from the central point (station) on the sheet toward each visible object.
The distance from the instrument station to each object is measured using a chain or tape.
These distances are then plotted to scale along each ray to mark the positions of the features.

This method is simple and quick, especially useful for small areas where the surveyor can see and reach all the required points from one location. It gives accurate results when the terrain is open and there are no obstacles.

Example: Surveying a park, playground, or open plot.

Advantages:

Fast and easy to perform.
Good for circular or fan-shaped areas.
Requires only one setup point if all objects are visible.

Limitations:

Not suitable if points are hidden or too far to measure.
Errors in distance measurement affect point placement.

Intersection Method

The intersection method is useful when distances to features cannot be measured directly. Instead, the position of a point is determined by drawing rays from two known stations. Here’s how it works:

The plane table is set at the first known station and oriented properly.
A ray is drawn from this station toward the object.
The table is then shifted to a second known station, again oriented, and another ray is drawn toward the same object.
The intersection of the two rays on the drawing sheet locates the object.

This method is very effective for surveying areas across rivers, valleys, or fences where the ground cannot be easily accessed for measurement.

Example: Plotting a tree or tower across a canal or pond.

Advantages:

No need to physically reach or measure the distant object.
Ideal for difficult or obstructed terrain.

Limitations:

Requires clear visibility from both stations.
Proper orientation is necessary to avoid errors.

Both methods make plane table surveying flexible and adaptable to different field situations. They are basic techniques that every surveyor should understand and apply based on the layout of the land.

Conclusion

Radiation and intersection are two useful techniques in plane table surveying. Radiation is used when all points can be seen and measured from one station, while intersection helps to locate points by sighting from two stations without needing direct measurement. These methods make fieldwork faster and more efficient in various surveying conditions.