The following corrections and calculations were performed.
(1) Ship magnetization correction
Hob = ARPYF + Hp ---(i)
Hob: Observed magnetic field vector (Ship coordinates)
A: Effect of induced magnetization of the ship
R: Matrix of rotation due to the roll
P: Matrix of rotation due to the pitch
Y: Matrix of rotation due to the heading
F: Geomagnetic field vector
Hp: Ship's permanent magnetic moment
Following the equation(i), we calculate the geomagnetic field F.
RPYF = BHob + Hbp ---(ii)
B: coefficient of Figure of 8 turn
Hbp: Permanent magnetic field vector of the ship
Reference: Isezaki,N., A new shipboard three-component magnetometer, GEOPHYSICS. VOL.51,NO10(1986);P1992-1998
(2) International Geomagnetic Reference Field (IGRF)
Synthetic geomagnetic field values are calculated from IGRF 11th Generation models by using navigation data ; latitude, longitude and date.
Reference:IAGA Division V-MOD Geomagnetic Field Modeling[
http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html]
(3) Calculation of the geomagnetic field anomaly
An = F - Figrf
An: Geomagnetic field anomaly vector
F: Geomagnetic field vector
Figrf: Synthetic geomagnetic field vector from IGRF
(4) Quality control of data
Following criteria were used for removal of data of low reliability:
・Time error (inversion of time, continuation of same timestamps)
・Summation of the difference of heading by one second exceeding 20 degree per 5 minutes
・Ground speed of the ship below 3knot or exceeding 20knot
・X, Y, or Z component of geomagnetic field anomaly exceeding ±4000nT
(5) Filtering of the geomagnetic field anomaly
Due to the residual undulation of the ship, a 120 second length Gaussian filter was applied for each component of the geomagnetic field anomaly data.
(6) Output of the data
Time (UTC)
Latitude (degree)
Longitude (degree)
X: Northward (positive on the north) component of geomagnetic field anomaly (nT)
Y: Eastward (positive on the east) component of geomagnetic field anomaly (nT)
Z: Vertical (positive for downward) component of geomagnetic field anomaly (nT)
T: Absolute value of geomagnetic field anomaly (nT)