Short Answer

Plane table surveying in civil engineering is a graphical method of surveying in which field observations and plotting are done simultaneously on a drawing sheet using a plane table. It is used to prepare maps directly in the field.

In this method, the surveyor observes points on the ground and draws them on paper at the same time. It is simple, fast, and does not require complex calculations. It is mainly used for small to medium areas where high precision is not required.

Detailed Explanation:

Plane table surveying basics

Plane table surveying in civil engineering is a type of graphical surveying method in which the field measurements and map drawing are done together at the same time. Instead of recording measurements first and drawing maps later, both activities are performed directly in the field.

In this method, a drawing sheet is fixed on a plane table, and the surveyor uses a sighting device to locate points on the ground. These points are then directly plotted on the sheet. This makes the process quick and reduces the chances of errors in transferring data.

Plane table surveying is widely used for small and moderately sized areas where high precision is not the main requirement. It is especially useful in preliminary surveys and mapping work.

Instruments used in plane table surveying

Plane table surveying uses simple instruments that help in field mapping. The main instrument is the plane table itself, which is a flat drawing board mounted on a tripod stand.

A drawing sheet is fixed on the table where the map is drawn. An alidade is used for sighting objects and drawing lines in the correct direction. It may be a plain alidade or telescopic alidade.

Other instruments include a spirit level for leveling the table, a plumbing fork with plumb bob for centering, and drawing tools like pencils and scales.

These instruments make it possible to observe and plot points directly in the field.

Methods of plane table surveying

There are different methods used in plane table surveying depending on the type of work.

Radiation method

In this method, all points are located from a single station. The surveyor places the plane table at one point and draws rays to all visible points. Distances are then measured and marked on the drawing.

This method is suitable for small open areas.

Intersection method

In this method, two different stations are used to locate a point. The point is fixed by drawing lines from two known positions that intersect at the required point.

This method is useful when distance measurement is difficult.

Traversing method

In traversing, the plane table is moved from one station to another. At each station, new points are plotted based on previous measurements.

This method is used for long areas like roads and boundaries.

Resection method

In this method, the position of the plane table station is determined by sighting known points on the map. It is useful when the exact location of the surveyor is unknown.

Advantages of plane table surveying

Plane table surveying has many advantages in civil engineering. It is simple and easy to use, even for beginners.

It allows direct plotting in the field, which saves time and reduces errors in calculation and drawing. It does not require complex mathematical work.

It is useful for small and medium areas and is suitable for preliminary surveys. It is also cost-effective because it uses simple instruments.

Limitations of plane table surveying

Plane table surveying also has some limitations. It is not suitable for large areas or very accurate engineering work.

It cannot be used in bad weather conditions like rain or strong wind because the drawing sheet may get damaged.

It requires clear visibility between survey points. It is also less accurate compared to modern surveying methods like total stations or GPS.

Importance in civil engineering

Plane table surveying is important in civil engineering because it provides quick and simple mapping of an area. It is useful in initial planning stages of projects.

It helps engineers understand the terrain and prepare rough maps before detailed surveying is done. It is widely used in rural surveys, road planning, and small construction projects.

It also helps in training students to understand basic surveying concepts.

Conclusion

Plane table surveying in civil engineering is a graphical method where observation and drawing are done together in the field. It is simple, fast, and useful for small areas. Although it has limitations in accuracy, it is still an important method for basic surveying and preliminary mapping work.