---
title: "Variations in magnetic declination"
subject: "Surveying I"
chapter: 4
key: "semester_3/surveying_i:variations-in-magnetic-declination"
source: https://www.bctengineer.com/q/semester_3/surveying_i/variations-in-magnetic-declination
site: BCT Engineering
---

# Variations in magnetic declination

Surveying I · Chapter 4: The Compass

- Asked in: 2062 Baisakh, 6 marks
- Video: [Magnetic Declination Explained | Your Compass Needle Does NOT Point North!](https://www.youtube.com/watch?v=SStmBzQC7eI) — TheTautLine
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/variations-in-magnetic-declination
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4

## Question

Write in brief on variations in magnetic declination.

## Short answer

Magnetic declination varies with time: secular (slow swing over decades), annual (about 1′–2′ per year), diurnal (daily swing of 1′–10′, with needle farthest east around 8 a.m. and west around 1–2 p.m.), and irregular (due to magnetic storms). For ordinary compass surveying, only secular variation must be accounted for when comparing old and new bearings.

## Answer

The magnetic declination at a place is not constant; it varies with time. The variations are:

| Variation | Nature | Amount |
|---|---|---|
| **Secular** | Slow swing of the magnetic meridian to the east and west of the true meridian over a long period (about 100–200 years) | Several degrees over many years; the main cause is changes in the earth's magnetic field |
| **Annual** | Small yearly change, in addition to the secular variation | About 1′ to 2′ per year |
| **Diurnal** | Daily swing about the mean position: in the northern hemisphere the north end of the needle is farthest east at about 8 a.m. and farthest west at about 1–2 p.m., then returns towards its mean position | About 1′ to 10′ in a day, depending on place and season |
| **Irregular** | Sudden, unpredictable changes caused by magnetic storms, earthquakes, volcanoes and aurora | Can be large (up to a degree or more) for short times |

**Effect in surveying:** because of these variations, bearings taken at different times cannot be compared unless the declination on each date is known. To re-survey an old line, the old declination is compared with the present one and the bearing is corrected. Diurnal and irregular changes are small and are neglected in ordinary compass survey, but secular change must be accounted for in work separated by many years.

## Check yourself

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

1. Secular variation is:
   A. A slow swing of the magnetic meridian over about 100-200 years
   B. A daily change
   C. A sudden change by magnetic storms
   D. A yearly change of 1-2 minutes
   Answer: A. Secular change is a long-period swing east and west of true north. (Key point: Secular variation)

2. Diurnal variation is:
   A. Change over centuries
   B. Daily movement of the needle, east in morning and west in afternoon
   C. Random change from storms
   D. Change due to iron objects
   Answer: B. The needle moves with the time of day. (Key point: Diurnal variation)

3. Irregular variations are caused by:
   A. The surveyor's tools
   B. Slow earth field change
   C. Magnetic storms, earthquakes, volcanoes, aurora
   D. Seasonal temperature
   Answer: C. They are sudden and unpredictable. (Key point: Irregular variation)

4. For resurveying an old line after many years, you must:
   A. Ignore declination
   B. Only correct for diurnal change
   C. Use FB only
   D. Compare old and present declination and correct the bearing
   Answer: D. Secular change must be accounted for when work is separated by years. (Key point: Surveying effect)

## Related in Surveying I

- [Define magnetic declination](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Adefine-magnetic-declination) — Surveying I, chapter 4
- [Quadrantal/WCB; declination, isogonic, agonic](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Abearings-declination-isogonic-agonic) — Surveying I, chapter 4
- [Corrected and true bearings, 5°E declination](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-true-bearings-5-deg) — Surveying I, chapter 4

## Related in other subjects

- [Domains, domain walls and magnetic losses](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adomain-walls-magnetic-losses) — Electrical Engineering Material, chapter 4