BARTINGTON INSTRUMENTS
Page 16 of 32 OM2020/5
Declination is calculated from the mean of the four observations ED, WD, EU, WU. The reference
observation for Declination is the Geographical Meridian (GM) obtained using a Circle Left /
Circle Right observation to eliminate optical collimation errors.
The azimuth reference for Declination may be surveyed relative to the reference position of the
theodolite.
Declination must be discovered before Inclination can be measured. Inclination is calculated
from the mean of the four observations SU, ND, SD, NU with the telescope aligned with the
calculated Magnetic Meridian. The reference observation for Inclination is the horizontal plane,
which is automatically derived from the fluid filled gravity compensator in the vertical reference
within the theodolite. Observations are made on the vertical circle, the plane of which must be
parallel to the Magnetic Meridian.
It is customary to record the time in Universal Time (U.T.) at which each of the eight
measurements is recorded. The mean time for D and for I can then be calculated subsequently
for record purposes.
6.1. Magnetic Hygiene
Note:
Before attempting magnetic observations, ensure adequate personal magnetic
hygiene. For example, garments should be free from steel fasteners or buckles.
Magnetic hygiene can be checked when commencing observations by moving hands, head and
torso close to the probe and observing any change in reading. Readings should not vary by more
than 0.2nT. In practice only the hand used for circle slow motion adjustment will be close to the
probe at critical times during measurement. Ultimate accuracy will depend on ambient magnetic
noise. The diurnal variation of the terrestrial field becomes most active at dawn and dusk.
6.2. Theodolite Set-up
For reference, see
and
Note:
When moving to a new site, allow at least 1 minute per 1K of temperature difference
to allow the theodolite to reach the ambient temperature.
To avoid ambiguous values for the magnetic meridian, place the theodolite on the pillar or tripod
such that the 0/360º horizontal circle transition will be clear of the four declination null point
readings.
6.2.1. Levelling the Theodolite
(See also the
.)
1. Use the three foot-screws to level the theodolite base using the circular level.
