.. _preproc-calibration-top:

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Automatic Calibration of Acceleration Data Using GGIR
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Cicada has implemented an algorithm adapted from the GGIR package to automatically calibrate acceleration data.

.. note::

    **Reference:** Van Hees, Vincent T., et al. "Autocalibration of accelerometer data for free-living physical activity assessment using local gravity and temperature: an evaluation on four continents." Journal of Applied Physiology 117.7 (2014): 738-744.

First, the algorithm aims to find data segments where there is (almost) no change in movement, i.e. where the standard deviation in the ``x``, ``y``, and ``z`` component of acceleration is less than ``sdCriterion = 0.013``. Then, the algorithm assumes that the mean acceleration for these stationary segments must be 1 g. Finally, the algorithm iteratively finds the new ``offset`` and ``gain`` factors for each acceleration direction such that its deviation from 1 g is minimized.

Please refer to the section 'Autocalibration method' with the methods section of the reference paper above for full details on this algorithm.

**To autocalibrate the acceleration data using GGIR,**

- click ``Preprocess`` > ``GGIR Automatic Calibration``.