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 allow 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 (settings) 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 > Calibrate e-obs IMU Data 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.
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.