Plane Table and Chain Surveying
Plane Table and Chain Surveying
This domain tests the plane table as a graphical survey method and chain surveying as a linear framework of triangles. Expect methods of plotting, orientation, resection, and the layout of base, check and tie lines.
Core rules
- Plane table methods: radiation (rays from one station), intersection (fix a point from two stations without measuring to it), traversing (plot a framework along a route), and resection (occupy the unknown point and sight known points).
- Orientation keeps the table parallel to its earlier position. Use a trough compass for magnetic orientation or a backsight along a plotted line for accurate orientation.
- In the three-point problem, the plane table station is found by resection to three well-defined points. Avoid the danger circle through the three points; if the station lies on it, the solution is unreliable.
- Chain surveying divides the area into triangles. The base line is the longest line and is measured most carefully; check lines and tie lines verify the framework. Offsets are measured perpendicular to the chain line, with ties for oblique offsets.
Worked example
A chain line AB is 80 m long. A perpendicular offset to a boundary is measured at 30 m chainage as 12 m and at 60 m chainage as 20 m. The area between the chain line and the straight boundary is a trapezium: area = 1/2 × (12 + 20) × (60 - 30) = 1/2 × 32 × 30 = 480 m². Add adjacent trapeziums or triangles for the full plot; do not treat the two offsets as a rectangle.
Traps
Do not confuse a check line with a tie line. Do not orient the table only once if the table is moved. In resection, a station on the danger circle gives a poor fix. Record offsets on the correct side (left or right) of the chain line; a mirrored offset ruins the plot.