MAGNETIC DECLINATION
- The magnetic poles are not points but oval areas located about
2,000 km away from the actual location of the geographic poles
of the earth.
- It is expected that the magnetic meridian and the true meridian
will not be parallel to each other. There are only few locations
on the surface of the earth where the two meridians coincide.
- The horizontal angle and direction by which the needle of a
compass deflects from the true meridian at any particular
locality is called the magnetic declination.
- The deflection of the needle may be eastward or westward of
the true meridian. See Figure 26-9 and 26-10.
TN
*
East
Declination
W
Magnetic
meridian
TN
MN

West
Declination
E
W
True meridian True
Meridian
Figure 26-9.
North end of needle is east
of true north.
Elementary Surveying Notes by AM Fillone
*
MN

E
Magnetic
Meridian
Figure 26-10.
North end of needle is west
of true north.
VARIATIONS IN MAGNETIC DECLINATION
The changes in direction of the magnetic meridian at any given
place are not constant. Variations in magnetic declination can be
categorized as daily, annual, secular, and irregular.
1. Daily Variation. This variation, also called diurnal variation, is
an oscillation of the compass needle through a cycle from its
mean position over a 24-hour period.
2. Annual Variation. Another form of periodic swing taken by
the magnetic meridian with respect to the true meridian.
3. Secular Variation. Secular variation is a slow, gradual, but
unexplainable shift in the position of the earth’s magnetic
meridian over a regular cycle. The meridian swings like a
pendulum in one direction for about 150 years and gradually
comes to a stop and then swings back in the opposite direction.
4. Irregular Variation. This type of variation is uncertain in
character and cannot be predicted as to amount or occurrence.
At high altitudes they usually amount to a degree or more.
Irregular variations are most likely to occur during magnetic
storms and disturbances associated with sun spots, and when
auroral displays occur.
ISOGONIC CHART.
A chart or a map which shows lines connecting points where the
magnetic declination of the compass needle is the same at a given
time.
Elementary Surveying Notes by AM Fillone