THE ADVENT OF DIGITAL COMPASSES: STATISTICAL EVALUATION AND COMPARISON WITH ANALOGUE COMPASSES

The advent of mobile devices, namely smart phones with iOS and Android operating systems, has put digital field data collection in the hands of almost everyone. The presence of internal sensors in these devices, such as accelerometers, gyroscopes, and magnetometers, led tech-savvy geoscientists to realize that smart phones could be used as digital compasses. The incorporation of GPS chips meant that these mobile devices had the potential to be useful tools for fast and efficient geologic mapping in the field. However, it was unclear whether digital compass apps could produce orientation measurements accurate enough for professional geologic field mapping.

In order to assess the validity of digital compasses on smart phones for measuring geologic features, we statistically analyzed and compared planar orientation data from a variety of Apple devices and Android devices using the Fieldmove Clino app, with analog measurements from Brunton Pocket Transits. Students at James Madison University and the University of Texas, El Paso collected data from both natural outcrops in the field, as well as plywood models in an indoor lab. Each student took multiple measurements of each planar feature with an analog compass as well as their digital compass app. The data was analyzed with Allmendinger's Stereonet program using a Fisher distribution analysis, which allowed for the geometric conversions of planar measurements to the pole of the plane, and both planar and linear measurements of the azimuth and elevation of vectors. Mean vectors and dispersion indices (k-values) were the result of the first analysis. Deeper analysis of fixed plane measurements in the lab using a Watson-Williams test indicates statistically significant differences between Brunton and smart phones, but not between iOS and Android devices. Furthermore, error distribution patterns for smart phones tended to follow a great circle, indicating potential precision problems with device azimuth measurements. Analysis of field-collected data are currently underway, but analysis of brand-new Brunton compass measurements also demonstrate considerable precision problems. This highlights the need for careful calibration and redundancy of measurements, regardless of device type.