- 2076 Chaitra · 4 marks
Methods of orienting plane table
Short answer
Plane table orientation uses trough compass (quick, for small work) or back‑sighting (reliable, for traversing). The four surveying methods: radiation (distance and ray from one station), intersection (two‑ray fix from base line, for inaccessible points), traversing (table moved and oriented along a route), and resection (locating the table station from known plotted points).
What are the methods of orienting a plane table? Describe the methods of plane table surveying with their salient features.
Methods of orienting the plane table
Orientation means placing the board at a new station so that all lines on the sheet are parallel to the corresponding ground lines. There are two methods:
- By trough compass (magnetic needle): the trough compass is placed along the magnetic meridian line drawn on the sheet at the first station, and the board is turned until the needle floats at the zero marks. Quick but affected by local attraction, so it is used only for small work or for rough orientation.
- By back-sighting: the alidade is placed along the line joining the present station to the previously plotted station, and the board is turned until the previous station is bisected. This is the more reliable method used in traversing and in intersection.
Methods of plane table surveying
The four methods are:
- Radiation - rays are drawn from one station to the points to be plotted and distances are measured and scaled along them.
- Intersection (triangulation) - a point is fixed by drawing rays to it from two stations at the ends of a plotted base line.
- Traversing - the table is set up at successive stations of a traverse; each new station is plotted by radiation and the table is oriented by back-sighting.
- Resection - the table station itself is located by drawing rays back from known plotted points to it (two-point and three-point problems).
Salient features
| Method | Salient features |
|---|---|
| Radiation | One station; rays and scaled distances; best for small, open areas |
| Intersection | Base line only; points fixed by two intersecting rays; for inaccessible points |
| Traversing | Table moved from station to station; for long narrow strips like roads and canals |
| Resection | Table station fixed by rays back to known points; two-point and three-point problems |
✓ Answer checked by Claude Opus 5.5 (high effort)
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