How is resection done in survey?

The process by which the position of an unknown points at which plane table is set is determined & plotted o the plan with the help of two or more well-defined station whose position are already plotted on the plan. In this method, the line of sight passes from unknown points to known points & the ray drawn is called as resector. 

• Let assume 2 pts A & B are plotted on a drawing sheet a & b.
• Now with help of a & b, we have to plot position of instrument 'c'.
• Now orientation of plane table at 'c' the intersection of two rays from a & b will give correct location of instrument.

Special methods of resection

1. Two pt problem
2. Three pt problem

 Two pt problem:- 

1. In this position, 2 well-defined pts whose positions have already been plotted on plan are selected.
2. Then by perfectly bisection these points, a new station is established at reqd positions.

Three pt problem:-

1. In this position, 3 well-defined pts whose positions have already positioned on map.
2. Then by perfectly bisecting these 3 well-defined pts, a new station is established at reqd position.
3. No auxiliary station is reqd in order to sol;ve this pblm.

 Errors & Precautions 

Instrumental error 

1. Surface of table may not be perfectly level.
2. The fiducial edge of alidade might not straight.
3. Vanes may not be vertical.
4. Needle of trough compass may not be perfectly balanced.

Personal error 

1. Levelling of table may not be perfectly level.
2. The table may not be perfectly centered.
3. The orientation of table may not proper. 
4. Object may not be bisected properly.
5. Rays might not drawn accurately.

Resection is nothing but free stationing. 

It is a method of finding a location of unknown point about known points. 

The total station simply means a zero point. 

Freely positioned the instrument hence every survey point should be at appropriate sight from the instrument. 

Then setting up a zero point on a known point and often it is impossible to see all survey points. 

With the zero points, the distances & bearings are measured to a minimum of two points of a control network. 

With the help of a geometric transformation, those polar coordinates are transformed into a coordinate system of a control network. 

Using square adjustment, errors are distributed. 

The orientation as well as the position of the total station about where the control network is established.

Thank You.

Resection Method. 

This method is used for establishing the instrument stations or after completing the traverse surveying the important object cannot be located due to difficulties & missed the station. 

PROCESS:- 

1. Select the two stations P & Q on the ground & measure the length PQ & plot to a scale pq on a suitable scale. 
2. Set the table at P , Centre, & level the table so that the point p is over the P. 
3. Place the alidade along pq, orient the table by turning the board until the signal at Q is bisected, & then clamp it. 
4. With the alidade touching at p sight the station R & draw a ray p to r. 
5. Shift the table at R. & make the center & level & sight the point P by back sighting & clamp the table. 
6. With the alidade centered on q, sight the station Q & draw a ray. The point of intersection of this ray & that previously drawn from p gives the reqd point r.