Structural data collection with mobile devices: Accuracy, redundancy, and best practices

Smart phones are equipped with numerous sensors that enable orientation data collection for structural geology at a rate up to an order of magnitude faster than traditional analog compasses. The rapidity of measurement enables field structural geologists, for the first time, to enjoy the benefits of data redundancy and quantitative uncertainty estimates. Recent work, however, has called into question the reliability of sensors on Android devices. We present here our experience with programming a new smart phone app from scratch, and using it and commercial apps on iOS devices along with analog compasses in a series of controlled tests and typical field use cases. Additionally, we document the relationships between iPhone measurements and visible structures in satellite, drawing on a database of 3700 iPhone measurements of coseismic surface cracks we made in northern Chile following the Mw8.1 Pisagua earthquake in 2014. By comparing phone-collected attitudes to orientations determined independently of the magnetic field, we avoid having to assume that the analog compass, which is subject to its own uncertainties, is the canonical instrument. Our results suggest that iOS devices are suitable for all but the most demanding applications as long as particular care is taken with respect to metal and electronic objects that could affect the magnetic field.