A murder scene, an auto accident, a collapsed building, a dilapidated stairway where a fall led to severe injury – Paul Francis has used laser scanning to document these and other scenes for forensic investigation and analysis. Last week we looked at how Francis and his team at Northway-Photomap Inc., Toronto, prepare for the scene visit, then go to the scene and capture it. This week follows the team through the rest of the process: assembling and analyzing the data, preparing and presenting final deliverables, and waiting for possible follow-up requests.
Data analysis
The first step in data analysis is to assemble the data from the firm's Optech ILRIS-3D scanner into a virtual scene. Northway-Photomap aligns and merges data from multiple scans using InnovMetric Software's PolyWorks. "In our police-sanctioned forensic work, the formation of a single virtual scene is the current final product," Francis reports.
But forensic projects are not always associated with police work. "We have numerous clients who are forensic engineers or insurance companies," according to Francis. "These are projects where the analysis of the data can become our work. Determination of slopes – on sidewalks, pool slides or roads – can be work brought back to your office." PolyWorks can be used to create PowerPoint presentations, AVI files, or more detailed analyses of the scan data – this varies according to the kind of project.
In traffic accidents, the depth of impact – the degree of vehicle deformation – indicates impact speed, and is a parameter needed for accident recreation. Also, Francis reports, "We have found that most policing agencies work in 2D – they want a single 2D line showing the outline of the vehicle." Adding the third dimension "allows for much more in-depth analysis" but often is not requested – "an open door for scanner operators," Francis believes. "When you start making deliveries to them, you have to go in and work with them to understand what [information] they're taking from an accident scene."
3D laser scan of stairway under load
Image courtesy Northway-Photomap Inc.
In one project Northway-Photomap scanned a set of stairs after an individual fell down them and become a quadriplegic. The client wanted the scene documented because the stairs were going to be torn out. "The stairs weren't in good shape – they were crooked, angled, and it was difficult for us to walk up them when we got to the scene." On the scene it occurred to the team to scan not just the stairs, but also how they behaved in use. "We scanned the stairs with no one on them, then we scanned them a second time with a 225-pound volunteer" standing on the treads. Processing the data in PolyWorks allowed Francis to show how far each tread flexed under load. "This was possible because the operator knew the ability of the software and hardware, and applied it to the situation on the scene."
Again, "this portion of the project is not defined until you arrive on the scene, with your knowledge of your system, and become a consultant to the client," Francis advises. "If this is a new client, make sure you keep them informed on the future uses you can see for the data."
