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.