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BARTINGTON INSTRUMENTS
Page 20 of 32 OM2020/5
and clamp both circles. Then use the vertical and horizontal circle adjustment controls to align
with the target.
1. Observe the target and record both the circle left (CL) and circle right (CR) horizontal circle
readings. Calculate the target datum (TD) as follows:
TD(±90°) =
(CL + CR)/2
Note:
this contains a ±90° component which will be subtracted when calculating D.
2. Set the vertical circle to 90º or 270º as appropriate, ensuring the micrometer scale reads
00’00’’ (see Figure 9).. Perform the four null mode magnetometer observations and
calculate the Magnetic Meridian mean value (MM).
Where MM =
(ED + WD + EU + WU)/4
3. Calculate the Declination (D)
Where D
=
MM + (TA) - TD ± 90º
Select the value expected for D and reject the larger value.
Note:
D will be positive if to the east and negative if to the west of GM. Observe correct use
of brackets and signs.
Figure 9. Magnetometer scale reading 00’00”
Example
(ideal observations):
If:
TA
=
-30º
CL
=
260º
CR
=
80º
EU, WD
=
14º
WU, ED
=
194º
TD(±90°)
=
(CL + CR)/2
=
(260º + 80º)/2
Micrometer
Scale