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
- Two pt problem
- Three pt problem
Two pt problem:-
- In this position, 2 well-defined pts whose positions have already been plotted on plan are selected.
- Then by perfectly bisection these points, a new station is established at reqd positions.
Three pt problem:-
- In this position, 3 well-defined pts whose positions have already positioned on map.
- Then by perfectly bisecting these 3 well-defined pts, a new station is established at reqd position.
- No auxiliary station is reqd in order to sol;ve this pblm.
Errors & Precautions
Instrumental error
- Surface of table may not be perfectly level.
- The fiducial edge of alidade might not straight.
- Vanes may not be vertical.
- Needle of trough compass may not be perfectly balanced.
Personal error
- Levelling of table may not be perfectly level.
- The table may not be perfectly centered.
- The orientation of table may not proper.
- Object may not be bisected properly.
- Rays might not drawn accurately.