Secular variation is:

A slow swing of the magnetic meridian over about 100-200 years

A sudden change by magnetic storms

Diurnal variation is:

Daily movement of the needle, east in morning and west in afternoon

Irregular variations are caused by:

Magnetic storms, earthquakes, volcanoes, aurora

For resurveying an old line after many years, you must:

Compare old and present declination and correct the bearing

Only correct for diurnal change

2062 Baisakh · 6 marks

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.

Write in brief on variations in magnetic declination.

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Variations in magnetic declination

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