--- title: "Plane table principles; orientation by back sighting" subject: "Surveying I" chapter: 6 key: "semester_3/surveying_i:orientation-back-sighting" source: https://www.bctengineer.com/q/semester_3/surveying_i/orientation-back-sighting site: BCT Engineering --- # Plane table principles; orientation by back sighting Surveying I · Chapter 6: Plane Table Survey - Asked in: 2078 Bhadra, 4 marks; 2075 Asoj, 4 marks; 2073 Shrawan, 4 marks; 2068 Baisakh, 4 marks (asked 4 times) - Video: [ORIENTATION OF PLANE TABLE BY BACK SIGHTING METHOD BY Easy civil](https://www.youtube.com/watch?v=D3HMAJyUT70) — EASY CIVIL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/surveying_i/orientation-back-sighting - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-6 ## Question What are the principles of plane table surveying? Describe the process of orientation of the plane table by back sighting with a supporting sketch. ## Short answer The principle of plane tabling is that rays drawn on the sheet from a station point towards objects are parallel to the corresponding ground lines. Orientation by back-sighting: at a new station, level and centre the table, place the alidade along the already plotted line to the previous station, then turn the board until the ranging rod at that station is bisected, and clamp. ## Answer #### Principles of plane table surveying **Principle:** the table is set up so that the rays drawn on the sheet from a station point towards the objects are *parallel* to the corresponding lines on the ground. The plotted figure is then similar to the ground figure. This needs three things at every station: 1. **Levelling** - the board is kept horizontal (spirit level on the board). 2. **Centring** - the point on the sheet is kept vertically above the ground station (plumbing fork and plumb bob). 3. **Orientation** - the board is turned so that all lines on the sheet stay parallel to their ground lines (by magnetic needle or by back-sighting a previously plotted line). #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is orientation by back-sighting preferred to the compass method? A. It needs no previously plotted line B. It does not need levelling of the table C. It measures the distance to the previous station D. It does not depend on the magnetic needle, which is affected by local attraction Answer: D. Back-sighting uses a plotted line and direct sighting, so local attraction does not affect it. (Key point: Why back-sighting is reliable) 2. What must already exist on the sheet before orienting at station B by back-sighting? A. The line ba, plotted from the previous station A B. A magnetic meridian drawn at station B C. The ray to an inaccessible point D. A closed triangle of error Answer: A. The method uses the previously plotted line ba as the reference direction. (Key point: Condition for back-sighting) 3. After placing the alidade along ba, what is done to orient the table? A. Turn the alidade until the needle is central B. Turn the board until the ranging rod at A is bisected, then clamp it C. Shift the tripod until the plumb bob touches A D. Draw a ray to B and measure its length Answer: B. With the alidade fixed along ba, rotating the board until A is bisected makes ba parallel to BA. (Key point: Procedure) 4. How is the orientation checked at the end? A. Measure the length ba again with a tape B. Read the compass bearing of ba C. Sight another known point; the ray should pass through its plotted position D. Repeat levelling with the spirit level Answer: C. A ray to a third known point passing through its plotted position confirms correct orientation. 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