rotation angles and geographic orientations in geomagnetic observatory
instruments using digital camera images
Merényi, L. (1), Hegymegi, L. (1), Domján, Á.
(1) Mingeo Ltd.,
At some types of
geomagnetic observatory measurements, it is crucial to accurately
measure the relative rotation angles and/or geographic orientations of
the magnetometer sensors. A typical example is the absolute
measurements using theodolite and a faraway geographic reference point
called MIRA. The required accuracy and resolution of such angle and
orientation measurements is very high, while the measuring device
should be non-magnetic, that complicates the instrument design and the
realization of the measurement.
Using appropriate digital image processing methods on images taken by
digital cameras, the movements of the recorded objects, including the
rotation of the objects can be estimated. We tested some possible
configurations and some digital image processing methods to measure
the relative rotation of a magnetometer sensor holder, and to relate
these relative measurements to reference directions in order to get
In this paper we present our tested methods and the results, including
the estimated resolution and accuracy. We also present some ways how
the method could be employed in absolute measurements.
There are some considerable advantages of the suggested image-based
procedure. Putting the camera away from the magnetic sensor, its
magnetic effect can be minimized. Moreover, there are more
possibilities to relate the rotation over the vertical axis to the
true North (e.g. in declination measurements), and to relate the
rotation over the horizontal axis to the vertical direction (e.g. in
inclination measurements). We tested the use of pictures taken from
sunlight shadow patterns in a horizontal plane, and the use of
pictures taken from free hanging strings. These pictures can give
natural horizontal and vertical reference directions for absolute
measurements. We tested some other methods to relate the measurements
to true North in the absence of the sunlight.
Limitations and errors due to the limited resolution and pixel noise
of the digital images can be efficiently reduced with some simple
digital computing methods. There are also some options to check and to
reduce the possible errors due to optical distortion of the camera, to
the non-ideal camera positions and to the long term small movements of
the parts of the camera measuring system.