Studying Steps: Forensic Gait Analysis

Studying Steps: Forensic Gait Analysis

In July 2000, forensic history was made when a jewellery thief was prosecuted with the help of forensic evidence. But this was not through DNA analysis or fingerprint comparison, as one might expect, but by studying the perpetrator’s gait. That is, how the individual moved. When podiatrist Haydn Kelly was called upon to offer his professional opinion in the case of a suspected jewellery thief, the expert was able to identify the suspect from video footage through nothing but the study of the individual’s walking mechanics, ultimately declaring that he believed the perpetrator captured in the video was the same as the person under suspicion.

Forensic gait analysis refers to the identification of an individual based on the characteristic features of the way in which they move. This sub-section of forensic podiatry (the study of evidence relating to the human foot) is based on the theory that people have unique movement characteristics as they move their body and limbs.

These characteristics may be subtle, for instance the angle of the feet throughout movement, or they may be fairly distinguishing, such as the condition of genu varum (bow-leggedness), which led to burglar John Rigg’s identification and subsequent conviction in 2008.

An individual’s gait can be affected by a range of factors, including sex, weight, height, age, but also taking into account external factors such as terrain, clothing and if they are carrying anything. Gait is a behavioural biometric and thus is subject to change based on a person’s emotional state or whether they are, for instance, in a rush to get somewhere quickly or under the influence of drugs or alcohol. It is worth nothing that the term ‘gait’ is not synonymous with walking, but refers to locomotion in general, whether that be walking, running, crawling, and so on. Furthermore, a person’s gait does not only relate to the movement of their feet and legs, as one might expect, but can also encompass their upper body movements, namely their arms and torso.

The repetitive gait cycle is said to be divided into two distinct phases; the stance phase, during which the lower limbs are providing support for the body by being in contact with the ground, and the swing phase, at which point the foot is no longer in contact with the ground. It is the spatial and temporal measurements throughout these phases that can be of great use to the gait analysis expert.

cctv

The Forensic Analysis of Gait

With the proliferation of closed circuit television (CCTV) cameras worldwide, it is becoming increasingly commonplace that a perpetrator is caught on camera in the act of committing of a crime. However this footage may not always be beneficial in identifying the subject based on their face. Perhaps the perpetrator has taken steps to conceal his or her identity or, more likely, the footage is simply not good enough for identification. A vast array of factors can affect the usefulness of CCTV footage, including camera quality, distance, lighting, environmental conditions and the angle of the recording. Thus all investigators may be left with is the blurry figure of their perpetrator. Not especially useful for identification purposes one might think.

So how can an expert use the study of a person’s gait to identify an individual? Well, it’s a little more complex than watching someone walk and deciding they look the same as a perpetrator captured on CCTV.

Identification in gait analysis is a comparative process – the footage of a perpetrator is compared with footage of a suspect walking and the similarities and differences are assessed. This analysis may be qualitative, involving the visual evaluation of movement by an expert, or quantitative, a far more complex process involving the collection, interpretation and comparison of numerical data relating to the movements.

The analysis of video evidence of a suspect typically involves the painstaking inspection of each frame of the video, identifying the slightest characteristic movement of the legs, arms, shoulders and head. A vast array of parameters will be studied throughout the analysis, including the length of steps and stride, speed and rhythm of walking, and finer points such as the angles of feet and hips as the individual moves. The analysis will additionally extend to any possible affecting factors, such as whether the perpetrator was carrying something substantial in the footage, such as a hefty bag of stolen goods, and how this might alter his or her gait. Any affecting factors such as these must be taken into account when recording footage of a suspect for comparison purposes. Footage of a person walking in light clothing and empty-handed is hardly a suitable comparison for a video of the perpetrator in a bulky coat and carrying a heavy rucksack.

gait

Although gait analysis is a relatively new approach in forensic science, the study of a person’s unique patterns of movement is far from novel. Gait analysis has been utilised in a medical setting for decades, focusing on the study of abnormalities in a person’s gait for the purpose of medical research and treatment. Similarly, gait has been studied in sports to aid athletes in perfecting their running technique, for instance. This history has allowed for the development of well-established procedures in studying and comparing the movement of individuals.

Technological Advances

Unsurprisingly, forensic gait analysis is not without its caveats, and experts in the field hasten to concede that it is not an indisputable field of study. Whereas DNA analysis and fingerprint comparison can, to an extent, provide clear evidence as to a connection, gait analysis is best utilised as a guideline as to the possibility of an identification being made. Although gait analysis is well-established in a clinical setting, the evidence available in a forensic setting is typically of relatively poor quality in comparison to that available in medical studies, thus footage may not actually be suitable for gait analysis in the first place.

The analysis of gait conducted by a human expert can obviously introduce the potential for human error and subjectivity, even calling the conclusions reached by experts into question, especially considering the lack of quality control in this field of study. A 2013 study concluded that people who were experienced in visual gait analysis were only able to consistently identify individuals based on their gait 71% of the time (Birch et al, 2013), indicating the scope for error.

Despite the obvious downfalls of gait analysis conducted by humans, the possibilities of gait as a form of biometric identification has sparked a great deal of research into the development of automatic gait recognition technology. Computer-based systems have already been developed that examine video frames and separate the silhouette of the individual in question from the background, then recording that individual’s movement. This, and similar systems, in theory allows for the detection and tracking of humans by using computer systems to store an individual’s ‘gait signature’ and later identify that person when they are next caught on camera. The development and use of automated gait recognition systems would reduce the scope for human error, greatly increase the speed of analysis, and allow for real-time identification based on gait. If sufficiently developed, such technology could even form the basis of new security systems, only allowing access to the unique gait of particular individuals, much like retinal or fingerprint scanners. Though gait analysis technology may still be in its infancy, the scope of potential applications is great.

References

BBC News. How can you identify a criminal by the way they walk? [online] Available: http://news.bbc.co.uk/1/hi/magazine/7348164.stm

Birch, I et al. Terminology and forensic gait analysis. Sci Justice. 2015(55), pp. 279-284.

Birch, I et al. The development of a tool for assessing the quality of closed circuit camera footage for use in forensic gait analysis. J For Legal Med. 2013(20), pp. 915-917.

Birch, I et al. The identification of individuals by observational gait analysis using closed circuit television footage. Sci Justice. 2013(53), pp. 339-342.

DiMaggio, J. A. Vernon, W. 2011. Forensic Podiatry: Principles and Methods. New York: Springer Science & Business Media.

Forensic Magazine. Considerations of Gait at Crime Scenes. [online] Available: http://www.forensicmag.com/articles/2014/08/considerations-gait-crime-scenes

Jasuja, O. P. Manjula. Estimation of stature from footstep length. Forensic Sci Int. 1993(61), pp. 1-5.

New Scientist. Cameras Know You by Your Walk. [online] Available: https://www.newscientist.com/article/mg21528835-600-cameras-know-you-by-your-walk

Stevenage, S. V et al. Visual analysis of gait as a cue to identity. Appl Cognitive Psych. 1999(13), pp. 513-526.

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Interview with Forensic Video Analyst David Spreadborough

DS

What is your field of expertise and what does working in this area involve?

I am a Forensic Video Analyst. In short, the forensic analysis of video involves the scientific examination, comparison and/or evaluation of video in legal matters. That though really only tells half the story. Video now comes from a multitude of sources and probably only a quarter of those could be classed as standardized. Most have some sort of proprietary nature, designed by the brand or manufacturer of a specific product. You probably think immediately of Video from surveillance systems; CCTV. However, there is also personal recording devices such as smart phones, digital cameras, body worn video and dash-cams. The majority of these are now digital and sit within the category of Digital Multimedia Evidence (DME). With CCTV, most still actually start off as an analogue video signal from an analogue camera and then the digitization gets done further down the line.

Many people have attempted to guess how many different types of digital video there are, with figures stretching from 3000 – 5000! As you can imagine, this then means that every job starts as research. Manufacturers of CCTV systems can change the format of their recording every time they upgrade that model of Digital Video Recorder. Same make and model number, but who knows how the digital video is encoded, stored and then decoded for playback.

It is imperative that the digital data is dealt with correctly. If not, it’s like the contamination of a piece of traditional evidence. If it is contaminated, any interpretation and/or enhancement of that footage could be questioned. Acquisition of the media comes first and this can be the most challenging part when it is stored inside a proprietary Digital Video Recorder. You can’t just ‘pull the drive’ and extract the data like Computer Forensics. Many machines don’t even store the data in a standard way. Then after acquisition, the playback and processing of that data is a minefield of contamination opportunities. If the video is processed incorrectly then the possibilities could be limited.

Identifying the correct method of acquisition, in order to preserve the integrity of the digital data is the start of my job. I can then ensure that it can be used effectively. From here comes the interpretation but I need to understand how it was originally captured and encoded. If I am to make a decisions based on what was recorded, I have to take these factors into consideration. To assist in my analysis I may need to conduct restoration and/or enhancement of the footage using digital image processing techniques. When the video has been understood and processed within a forensic workflow, I am able to prepare that footage for comparative analysis or presentation. All of this has to be conducted in a manner that is repeatable and acknowledged by the community.

How did you come to be involved in this line of work?

I had been a Police Officer for 12 years, 6 in the Met and then 6 in Cheshire. As a tutor constable, I became frustrated over the methods and processes involved when dealing with CCTV evidence. As a result I put forward a suggestion to my Divisional DI with a solution of how CCTV could be dealt with, not only more effectively, but with higher evidential integrity. I already had some understanding of digital video so the initial stages were made a little easier. Within a few weeks, one of the first Divisional CCTV Units in the Country was started. That was back in 2003, and although technology has brought about huge change and increased our capabilities, the reasons why I started that unit remain. With the increased reliance on video evidence, but with limited budget and resources, those reasons are actually more important today.

What do you think is the most common misconception about your field of work?

“It’s just a Video”.

“Don’t do it that way, it takes too long”.

“Don’t worry about it, it’s only a DVD”.

“Just press, ‘Print Screen'”.

Senior Officers and Managers see how easy it is to stick a dancing cat on YouTube and then, for some unknown reason, apply that logic to evidence. Evidence that is controlled by guidelines and law. You can’t cut corners with evidence. The misconception comes from how easily video and images can be manipulated. It is the analyst’s responsibility however to ensure that the media is interpreted correctly and all processing is completed adhering to the rules of evidence. When corners are cut in order to speed things up and balance the spreadsheet, that’s when things can go wrong. It’s not just a picture or a video. It is evidence and should be treated as such.

When CCTV is dealt with correctly, the value stretches throughout the Criminal Justice System. From recognition intelligence through to impactive evidence used to exonerate or convict. I have heard some managers class CCTV as ‘Low Yield’, referring to the fact that they regard the expense of a Forensic Imaging Unit as needless. It’s a classic case of ‘you get out what you put in’. If you don’t invest and use the full capabilities available, it’s common sense that you won’t see the results.

Do you think the field of forensic video analysis in the UK has been affected by the recent changes to forensic science services?

It was a really tough decision to leave the Police after 24 years of service. I did so though because I care. I cared about the quality of my work. I cared about doing it right. I cared about making the right decision. It’s only been a few months so it’s a little difficult for me to answer with personal experience.

What I can say though, is that in both the Police and private sector there is pressure now to ‘dumb it down’. Ensuring that more can be done for less. Rather than understanding what the evidence is, and how it should be processed, pressure is put on technicians to get the jobs done. I have seen countless examples where incorrect processing has been completed, resulting in images and video being presented that are not a true and accurate representation of the events captured by that camera. I have seen many comparative analysis reports where the images being used have not been created correctly according to the Digital Imaging Procedure. I have seen Video presented that has been acquired by methods not even mentioned in the Home Office Guidelines for Retrieval of CCTV.

Some of these issues end up causing further delays to an investigation. If I receive a disk from a solicitor, and am under instruction to analyze the footage, there is often disbelief when I re-contact to state that it is not the original evidence. They then have to request the original evidence and the whole process starts again. The introduction of standards in Forensic Video Analysis through ISO will help, but we are a long way off.

The majority of Forces invested when the FSS closed, ensuring that only the most complex or time consuming tasks required out-sourcing to the private sector. A few of the Forces, with experts in-house, were also able to make the correct decisions on out-sourcing as they knew the technical possibilities involved and the expected results. With current budgetary restraints within the legal system, this is reversing. Forces just have not got the money or the staff do the jobs anymore.

Do you think the specialty of forensic video analysis is currently being utilised to its full potential?

Not at all. Well, actually there are one or two places in the UK where it is given the respect, and then the funding, that it requires but there are many places where it is simply not possible to develop, due to a lack of joined up thinking. There are many facets to DME and Forensic Video Analysis. When a number of these get linked, then everybody wins. From reviewing CCTV during a major incident, the recovery of data, the use of Social media for intelligence… I could go on.

I go back to my answer earlier, when I stated that I have heard it called ‘Low Yield’. Think of a field of Barley. Left alone, it will produce a bad crop with a low yield. Invest the time and effort and you will get a good crop, with a high yield. To fully reach its potential, you need someone who knows what they are doing and is prepared to invest.

Do you have any advice for those wishing to follow your career path?

It’s an unknown world to most! People often think that video is just moved magically across networks and ‘it is what it is’. As a result, it’s one of those jobs that you don’t really know exist. It is also a very small, niche sector of Digital Forensics. All the kudos and media navigate to anything with Cyber and Security in the title… Forensic Video Analysis is tucked away! You do see the jobs advertised every now and again, within Police Forces and also within the Private Sector Forensic services.

Enthusiasm, a willingness to learn… and then learn some more, along with a ‘never give up’ attitude. Investigating proprietary video can be extremely frustrating. That was one of the reasons why I started my blog over 6 years ago. I needed a way to document some rather lengthy processes and in doing so help others. I strategically left the comments enabled so people could add in their solutions to problems. I quite often have to go back and carry on research with a file first encountered three years ago!

Always ask questions of the video. Look beyond the image to understand how that video was constructed. Only then will you be able to interpret that video correctly. Will the compression affect enhancement? If you don’t understand the compression – are you enhancing artefacts that were never present in the scene? A lot of young people now come to it through traditional computer forensics. Some people through digital imaging and photography. Whatever route is taken, never be afraid to ask for help. The worldwide community is pretty small but with some very great people. It is rare to come across something that someone else has not encountered before.

The career will take you from just attempting to get a video to play (easier said than done sometimes), through a multitude of tasks and requirements. You could end up doing 3D Laser scanning of scenes and overlaying the CCTV Footage with aerial footage from a drone. You could end up data carving huge amounts of video and reverse engineering the embedded timecodes. You could end up conducting advanced Digital Imaging techniques to enhance details in a video not originally view-able.

If you enjoy working with digital media and want to be challenged every day, then Forensic Video Analysis may be something to look at… (excuse the pun!).

Is there anything else you’d like to add?

FVA is now part of me. It’s who I am. It’s not just a job or a career…. it’s my passion. Every case is different and requires thinking. This is not in the ‘Push Button Forensics’ domain.  It’s been a pleasure being interviewed and I hope that it has opened up a few people’s eyes to the world of FVA.

Website: www.spreadys.com

Twitter: https://twitter.com/spreadys

 

If you’re a forensic scientist (academic or industry) or a crime scene investigator and would like to be part of this series of interviews, get in touch by emailing locardslabblog[at]gmail.com.

This is Part 5 of our series of interviews with forensic professionals.

Interview Series Part 1 – Interview with Forensic Identification Scientist Alexandre Beaudoin

Interview Series Part 2 – Interview with Forensic Expert Robert Green OBE

Interview Series Part 3 – Interview with Forensic Expert & Consultant Gareth Bryon

Interview Series Part 4 – Interview with Forensic Identification Specialist Donna Brandelli