--- title: "Orientation in plane tabling; reliable method" subject: "Surveying I" chapter: 6 key: "semester_3/surveying_i:orientation-more-reliable-method" source: https://www.bctengineer.com/q/semester_3/surveying_i/orientation-more-reliable-method site: BCT Engineering --- # Orientation in plane tabling; reliable method Surveying I · Chapter 6: Plane Table Survey - Asked in: 2080 Bhadra, 1+2 marks; 2079 Bhadra, 1+3 marks (asked 2 times) - Video: [Plane Table Surveying: ORIENTATION BY BACK SIGHT METHOD](https://www.youtube.com/watch?v=9lZEZ8RIafw) — Er Joga Singh Civil Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/surveying_i/orientation-more-reliable-method - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-6 ## Question What is orientation in plane table surveying? Describe the more reliable method of orientation (with a sketch) and say why it is more reliable. ## Short answer Orientation is the process of keeping the table aligned so that plotted lines stay parallel to ground lines. The reliable method is back-sighting: at a new station, alidade is set along a plotted line towards the previous station, and the board turned until the rod at that station is bisected. It is more reliable because it does not use the magnetic needle, which is affected by local attraction. ## Answer **Orientation** is the process of putting the plane table in a fixed direction at every station, so that all the lines on the sheet are parallel to the corresponding lines on the ground. It is needed whenever the table is shifted from one station to another. #### Orientation by back-sighting This is the more reliable method, because it does not depend on the magnetic needle, which is affected by local attraction. **Condition:** the table is at station B; the line ba (from the previous station A) is already plotted. 1. Level the table and centre **b** over station B. 2. Place the alidade along the line **ba**. 3. Turn the board until the ranging rod at A is bisected by the line of sight. 4. Clamp the board. The table is now oriented, since ba is parallel to BA. 5. Check by sighting another known point; the ray should pass through its plotted position. ```text SHEET (table at B) GROUND a o--------o b A o ........ o B \_ alidade on ba (rod at A seen along the line) along the line) ``` The board is turned about its vertical axis, with the alidade kept on the line ba, until the rod at A is bisected. #### Why back-sighting is more reliable - It depends only on the direction of a line already plotted on the sheet, and the line is sighted directly through the alidade. - The magnetic needle (the other method) is affected by local attraction from iron, steel structures, power lines and by its own errors, so orientation by compass may be wrong by several degrees. - A long back-sight line gives a more accurate orientation than a short one, and the result can be checked by sighting a third known point. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is orientation in plane table surveying? A. Placing the table so that all lines on the sheet are parallel to the corresponding ground lines B. Making the board horizontal C. Placing the sheet point over the ground station D. Measuring the base line Answer: A. Orientation keeps sheet lines parallel to ground lines whenever the table is shifted. (Key point: Orientation) 2. Which method of orientation is more reliable? A. Trough compass B. Back-sighting a previously plotted line C. Orientation by eye D. Plumbing fork Answer: B. Back-sighting does not depend on the magnetic needle. (Key point: Reliable method) 3. Why can compass orientation be wrong by several degrees? A. The sheet shrinks B. The alidade is too short C. Local attraction from iron, steel structures and power lines, plus needle errors D. The plumb bob swings Answer: C. The needle is disturbed by magnetic material nearby. (Key point: Why unreliable compass) 4. Which back-sight line gives a more accurate orientation? A. A very short line B. A line drawn by eye C. Any line, length has no effect D. A long line, checked by sighting a third known point Answer: D. A longer line reduces the angular effect of sighting errors, and a third point gives a check. (Key point: Back-sight length) ## Related in Surveying I - [Reliable orientation; intersection over radiation](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aorientation-reliable-intersection-vs-radiation) — Surveying I, chapter 6 - [Plane table principles; orientation by back sighting](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aorientation-back-sighting) — Surveying I, chapter 6, asked 4× - [Two-point resection in plane tabling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-point-resection-plane-table) — Surveying I, chapter 6 - [Advantages and disadvantages of plane tabling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aadvantages-disadvantages-plane-table) — Surveying I, chapter 6, asked 2× - [Intersection versus resection in plane tabling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aintersection-vs-resection-plane-table) — Surveying I, chapter 6 - [Plane and geodetic surveying; principles](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aplane-geodetic-and-principles) — Surveying I, chapter 1, asked 7× ## Related in other subjects - [T/F: radial more reliable than network](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-radial-more-reliable-network) — Transmission and Distribution System Design, chapter 5, asked 2× - [Making programs more readable](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Areadable-programs) — Instrumentation II, chapter 9 - [Spatial orientation / normal of a polygon](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aspatial-orientation-polygon-calculation) — Computer Graphics, chapter 6, asked 5×