Q.11 In plane table surveys, the orientation of the table only by back sighting is preferred when
- Athe traverse is too long
- Bthere is no second point for orientation
- Cspeed is more important than accuracy
- Dthe plane table can be set on a point already plotted on the sheetCorrect
Solution
Concept
Back sighting is possible only when the table stands over a station that is already plotted and the line to a previous plotted station is known.
Step-by-step solution
1. Orientation means making plotted lines parallel to the matching ground lines. 2. In back sighting, the table is set over a station already plotted on the sheet, for example B, with b over B. 3. The alidade is kept along the plotted line ba, the table is turned until A is bisected, and then it is clamped. 4. This is possible only when the occupied station is already plotted on the sheet. 5. So option D is correct.
Explanation
Back sighting is the most accurate method of orienting a plane table. It needs the table to be set over a point that is already plotted, so the alidade can be laid along an existing plotted line. If the station is not plotted, resection is used instead.
Why the other options are wrong
A: In a long traverse, small orientation errors add up, so length is not the condition for this method. B: Back sighting still needs a previously plotted station to sight back to. C: Compass orientation is faster but less accurate; back sighting is chosen for accuracy, not speed.
Shortcut
Back sighting needs a plotted station under the table. Resection is used when the station is not plotted.
Common mistake
Confusing back sighting with resection. Resection is used when the occupied station is NOT plotted.
Correct answer: D) the plane table can be set on a point already plotted on the sheet