---
title: "Reliable orientation; intersection over radiation"
subject: "Surveying I"
chapter: 6
key: "semester_3/surveying_i:orientation-reliable-intersection-vs-radiation"
source: https://www.bctengineer.com/q/semester_3/surveying_i/orientation-reliable-intersection-vs-radiation
site: BCT Engineering
---

# Reliable orientation; intersection over radiation

Surveying I · Chapter 6: Plane Table Survey

- Asked in: 2074 Chaitra, 4 marks
- Video: [Methods of Plane Tabling: INTERSECTION METHOD](https://www.youtube.com/watch?v=km4cII6d9l0) — 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-reliable-intersection-vs-radiation
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-6

## Question

Describe the reliable method of orientation of plane tabling and the circumstances in which the intersection method of plane tabling is more preferable than radiation.

## Short answer

The reliable orientation is by back-sighting: at a new station, the alidade is placed along the plotted line to the previous station, and the table turned to bisect that station's rod. Intersection is preferred over radiation for inaccessible points where distances cannot be measured, when many distant points are to be plotted quickly, and when a check on location is needed, since only a base line is measured.

## Answer

#### Reliable method of orientation
Orientation by **back-sighting** is the more reliable method. It is not affected by local attraction, unlike the magnetic needle.

1. At the new station B (line ba already plotted from the previous station A), level the table and centre **b** over B.
2. Place the alidade along the line **ba**.
3. Turn the board until the ranging rod at A is bisected, then clamp it.
4. The sheet is now parallel to its first position. Check by sighting a third known point; the ray must pass through its plotted position.

#### When intersection is preferred over radiation
| Intersection | Radiation |
|---|---|
| Used when the point is inaccessible (across river, valley, swamp) so distance cannot be measured | Needs the distance to the point to be measured with tape/chain |
| Only the base line is measured | Every ray needs a measured length |
| Many distant points can be plotted quickly | Suitable for points near the station |
| Third ray gives a check | No check on the plotted point |
| Used for large areas and long distances | Used for small areas |

So intersection is preferred when the distance to the point cannot be measured, when the points are far, when many points are to be located quickly, and when a check on the position is required.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which orientation method is reliable and why?
   A. Back-sighting, because it is not affected by local attraction
   B. Compass, because it is faster
   C. Eye, because it needs no instrument
   D. Compass, because it uses the plotted line
   Answer: A. Back-sighting does not use the needle. (Key point: Reliable orientation)

2. When is intersection preferred over radiation?
   A. When the point is very near and easily chained
   B. When the point is inaccessible and its distance cannot be measured
   C. When the area is small and open
   D. When no base line is available
   Answer: B. Radiation needs measured distances; intersection does not. (Key point: When intersection preferred)

3. What is measured in intersection?
   A. Every ray length
   B. Distances to all points
   C. Only the base line
   D. Nothing at all
   Answer: C. Points are fixed by rays, so just the base line is taped. (Key point: Measurement)

4. What gives a check on a point located by intersection?
   A. A compass bearing
   B. The paper size
   C. Repeating the same ray
   D. A third ray from another station
   Answer: D. If the third ray passes through the same point, the plot is verified; radiation has no such check. (Key point: Check)

## Related in Surveying I

- [Orientation in plane tabling; reliable method](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aorientation-more-reliable-method) — 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
- [Radiation and intersection methods](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aradiation-and-intersection-methods) — 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
- [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×
- [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×

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- [When terminal voltage is less or more than emf](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-less-or-more) — Electric Machines II, chapter 1
- [RE: no more than 3 a's](https://www.bctengineer.com/question.md?key=semester_3%2Ftheory_of_computation%3Are-at-most-3-as) — Theory of Computation, chapter 2
- [Why 3D graphics is more complex than 2D](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Awhy-3d-more-complex-viewing-pipeline) — Computer Graphics, chapter 4