SAM-III Construction Manual
See last page for document and copyright information, File: SAM-III Magnetometer Construction.doc, Page 64
Also, probably one of the best references on the Earth’s magnetic field ever written is
Geomagnetism
,
Vol. I and II, by S. Chapman and J. Bartels. This 2-volume set originally was published in 1940 and is
available from online used-book sellers.
Sensor body direction
For correct measurement of the magnetic field, the sensor body and pins must be oriented as shown in
the table below. If the orientation is backwards, changes in the measured induction will be displayed
opposite to actual (for example, a positive change will be displayed as a negative change).
Magnetic field component
Sensor body orientation
Pins direction
X
North – South
SOUTH
Y
East – West
WEST
Z
Vertical
UP
When sensors are setup to measure the X- and Y-components, the sensors are installed perfectly
horizontal. When a sensor is setup to measure the Z component, it is installed perfectly vertical. In an
installation with more than one sensor, all sensors are rigidly and accurately mounted at right-angles to
each other. However, the sensors must be physically separated as discussed in the previous section.
The rotational orientation of the sensor’s longitudinal axis does not matter. Once its longitudinal axis is
pointed properly, the body can be rotated and it will not change the measurements.
Coordinate Systems
One of three coordinate systems is used in professional observatories, two based on geographic
coordinates and one on geomagnetic coordinates:
•
Geographic coordinates – North (X), East (Y), Vertical down (Z)
•
Geographic coordinates – Horizontal intensity (H), Declination (D), Vertical down (Z)
•
Geomagnetic coordinates – Magnetic north (H), magnetic east (D), Vertical down (Z)
The geographic coordinate systems use true north as reference, and the geomagnetic coordinate system
uses magnetic north as reference. The one you chose is up to you, but the advantage of using
geographic coordinates is that you always know what coordinate system you are in. The geomagnetic
coordinate system drifts with time - if you perfectly align one of your sensors with magnetic north today,
it will be out of alignment tomorrow. Refer to the Geomagnetism Tutorial for additional information.
The standard for the SAM Magnetometer Network project is
geographic coordinates
, X, Y and Z, with
reference to true north. This means the N-S sensor is installed with the cylinder axis horizontal and
pointed toward true north. The E-W sensor is installed with the cylinder axis horizontal and pointed 90
degrees with respect to true north. The vertical sensor is installed with the cylinder axis pointed
vertically. The sensor pins must be oriented as discussed above.
