by Jo Ellen Nott
In a stabbing case in The Hague, Netherlands, a suspect facing charges in the deadly incident denied his involvement. Wanting to prove his presence at the crime scene, the Dutch police turned to digital evidence and, more specifically, data from the suspect’s cellphone.
The police were able to use location tracking and step counters to follow the suspect’s movements up to the front door of the building. They then encountered a glitch – the suspect’s phone only registered two floors, not the third-floor crime scene. To make sense of the seemingly contradictory finding, the Dutch police asked the Netherlands Forensic Institute (“NFI”) to evaluate the accuracy of iPhone’s building level registration.
To comply with law enforcement’s request, NFI researcher Jan Peter van Zandwijk and a student from the Amsterdam University of Applied Sciences conducted experiments using various iPhone models. Testers walked stairs at the NFI building in different sequences and speeds, wearing the phone in different positions to simulate real-world scenarios.
Their experiments revealed a significant detail – the iPhone’s health app logs an elevation of approximately three meters as one floor or building level, but only when the phone’s owner is walking. Elevator rides and standing still on escalators do not collect altitude data. The NFI researchers also discovered that the health app only registers floors when you walk upstairs. There is, however, a temporary data file that contains information about stairs walked both up and down. This file is not visible to the user but can be retrieved from a telephone using forensic software.
Van Zandwijk’s research showed that the iPhone registers every three meters of altitude difference as a floor, but not physical building levels. In an important detail, the researchers found that the accuracy of registration is dependent on elevation changes – if the difference is less than three meters, reliability diminishes. In the stabbing suspect’s case, the two floors registered on his phone matched the scenario of walking from the ground to the third floor. Since the third floor was seven meters high, it was the equivalent of two three-meter distances of altitude plus an additional meter.
Van Zandwijk emphasized that the number of registered floors on a phone do not necessarily correspond to the actual physical floors. This finding is valuable in crime reconstructions because it allows investigators to evaluate a suspect’s claims with greater accuracy and verify his or her movements.
This study of the reliability of altitude data on an iPhone is part of a broader NFI project examining the correlation between physical actions and digital tracking, a state-of-the-art research field in the international scientific community. Before this study, NFI research focused on the reliability of step and distance registration in the iPhone health app.
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