FUNDAMENTALS OF SURVEYING : Variation in declination

Variation in declination:

> Declination at a place varies from time to time. (never remains constant)

> There can be four types of variation in declination

a) Diurnal variation / daily variation

> It is the systematic variation of declination from its mean value during a period of 24 hours. > The difference between morning and afternoon declination is as much as .

> Extent ofdeclination depends upon

1)        Locality / place : more at magnetic poles and less at equator.

2)        Season: more in summer than in winter.

3)        Tine : More in day and less in night.

4)        Daily variation amount changes from year to year.

b) Annual variation

> Variation over a period of year.

c) Secular variation:

> Variation over a long period generally about 250 years.

> Follows a roller – coaster (sine curve) pattern and swings like a pendulum.

> It is most important in the work of survey or

> In Paris records shows a variation from 11⁰ E in 1680 to 20⁰ W in 1820.

d) Irregular variation

> Due to magnetic storms, earthquakes and other solar influences

Determination oftrue bearing:

> All important surveys are plotted with TM since direction of magnetic bearing changes from

time to time.

True bearing = Magnetic bearing declination

+ , if declination is to the right or East of TM.

-, if declination is to the left or west of TM.

> Above rule is for only WCB.

> If RB are given, then diagrams should be drawn.