NFI scientist Jan Peter van Zandwijk. Credit: NFI
A deadly stabbing incident on the third floor of an enclosed flat building in The Hague. A suspect denying any and all involvement.
To determine whether the suspect’s presence at the crime scene can be established, the police examined the digital traces on his telephone. Using location tracking and the step counter, this proved successful—up to the front door of the building. Not inside, on the third floor. That’s because the suspect’s phone only registered two floors.
The police then contacted the Netherlands Forensic Institute (NFI) asking: how reliable is the registration of building levels on an iPhone?
Crime reconstructions
NFI scientist Jan Peter van Zandwijk and a student at Amsterdam University of Applied Sciences conducted experiments to test the reliability of building level registration.
“We did tests with different iPhone models,” said Van Zandwijk. “We had several testers walk up and down stairs multiple times. Sometimes they walked up one staircase, sometimes three, then one down, in a variety of sequences and at different speeds. The testers also wore the phone in different places each time.”
The scientist conducted the experiments to gain insight into the factors that impact the accuracy of the registration in order to make a well-founded statement as to reliability.
The iPhone’s health app states that an elevation of approximately 3 meters is registered as one floor or building level. The NFI research demonstrates a vital precondition: you must be walking at the same time, as the phone does not register floors/building levels when you are not walking.
“When you gain altitude in a lift, the app does not register floors,” said Van Zandwijk. “Nor does it when you are standing still on an escalator, but it does register the floors when you are walking on an escalator.”
The researchers found a technical file on the iPhone that contains temporary 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.
The NFI research shows that an iPhone does indeed register roughly every 3 meters of altitude difference as a floor. It does not count physical building levels, in other words.
“We measured the height of the floors at the NFI. In our research, we noticed that the number of floors registered by the telephone most frequently coincides with the number of 3-meter blocks that a tester walks up or down one or more floors. If the elevation varies less than 3 meters, registration is less reliable. We saw that the app sometimes does, and at others does not, register a floor if the elevation is 2.4 meters. So, the result depends on the specific situation,” said Van Zandwijk.
Research at the crime scene
In addition to the reference research at the NFI, the investigators did additional research at the crime scene.
“We measured and tested everything there, too. We examined what the iPhone registers in terms of floors in the scenario where you walk from the ground floor to the third floor or a different floor,” the researchers said.
Van Zandwijk explains that looking at the number of meters of elevation, the registration of two floors in the suspect’s phone matches the scenario in which he walked from the ground floor to the third floor. This is because the third floor was about 7 meters up—i.e. two 3-metre blocks and another 1 meter.
“We know that the app counts every 3 meters as one floor. As such, the number of physical floors does not have to match the number of floors registered by the phone,” said Van Zandwijk.
Other cases
The NFI’s study on the reliability of building level registration helps the police in crime reconstructions. What did a person do in a specific timeframe? Did they walk upstairs or did they only walk on flat surfaces? Even if a suspect offers an alternative scenario of what may have happened, the information can be tested.
The study is part of a bigger project in which the NFI examines the relationship between actions in the physical world and digital traces. Internationally speaking, such research is relatively new. Prior to this, NFI specialists studied the reliability of step and distance registration of the iPhone health app.
A scientific publication on this research will be published in the near future.