# IMU magnetic field data

The orientation of tags in 3D space is a non-trivial problem that **cannot usually be resolved via acceleration sensors**
alone. Tags by e-obs and other manufacturers therefore integrate data gyroscope and compass data
for state-of-the-art estimates of tag orientation termed **IMU** (**intertial measuement unit**).

This type of data can be imported into Firetail via `File > Open CSV`

or via the Movebank Data interface.
When working with IMU data it is important to

**calibrate the gyroscope data**in the context of compass measurements- understand and interpret the outcome (quaternion data)

Note that the outcome of e-obs tags is quaternion data that allows for a direct and unique computation of rotations
in 3D space. As their visual interpretation is difficult, **Firetail offers an Euler angle visualisation** and a
**“Quat-Preview”**, i.e., a 3D model of a tag or a bird (adjustable in the settings dialog) following the measured orientation.

The preview uses common terms from flight cockpits namely

**roll**: rotation along the “head-tail” axis**pitch**: rotation along the “wings” axis (upwards vs. downwards)**yaw**: rotation along the “back-chest” axis (view direction)

## Calibration

The raw data **must be calibrated first** to acquire sensible orientation values.
Use `Data > IMU: Calibration`

to fit an appropriate model [1] and calibrate the data accordingly.

Check *Restrict to Selection* to calculate the MAG values only for the currently selected sections.
By default, the complete time-span is computed.

Check *Clear Previous Calibration and Fusion Results* to remove previous results.

Choose among *one-pass* fitting or *multi-pass* fitting up to a defined error threshold, i.e. an iterative
refinement of the initial model. The magnetometer data is fitted to an ellipse in 3D space whereas the number
of data with more than k (max percent error parameter) percent deviation is treated as an outlier.

### Interactive Calibration

In many cases the `goodness of fit`

for the calibration of the underlying model should be estimated. In the calibration dialog

- click
`Interactive Selection`

to open the magnetic field calibration window. - use the burst view of the main window to select a time range
- the calibration preview is updated interactively as well as the solution matrix and origin (zero)
- the number of outliers from the ellipsoid fit indicates the goodness of fit and correlates with model quality

To apply the model, press `OK`

. To see the result for the selection range, toggle `Restrict to Selection`

.

### Sensor Fusion

*Sensor Fusion*: multiplicative integration factors for compass fusion. This should usually not be modified. Upcoming
versions will include visual error inspection such that the effect of these parameters is more interpretable.

### Output

Checkbox | Description |
---|---|

Calibrated Magnetic Field | create a lane for calibrated magnetic field data |

Fusion as Quaternions | create a lane for quaternion data used for the computation of rotations |

Fusion as Euler Angles | create a lane for a interpretable Euler angle graph of rotations |

## Visualisation

Given data from an IMU (inertial measurement unit) is present, *Firetail* offers a visualization option
to show the spatial orientation of the (e-obs) tracking device.

You will now see the device orientation in a 3D viewport:

Note that in the settings you may choose among a bird and e-obs device visuals.

## References

- Li, Q., & Griffiths, J. G. (2004, April). Least squares ellipsoid specific fitting. In Geometric modeling and processing, 2004. proceedings (pp. 335-340). IEEE.