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Determination of rotation angles and geographic orientations in geomagnetic observatory instruments using digital camera images |
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Merényi, L. (1), Hegymegi, L. (1), Domján, Á. (1) |
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(1) Mingeo Ltd., Budapest, Hungary |
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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 geographic orientations.
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. |
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Instrumentation |
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