西约克郡警察的成像团队因其创新的3D激光扫描和CCTV技术而实现了一系列刑事定罪。
丹·夏普（Dan Sharp）看到西约克郡警察正在为犯罪现场测量师做广告时，他正在担任电气测量师，并引起了他的兴趣。他申请，找到了工作，再也没有回头。今天，他是韦克菲尔德（Wakefield）的高级成像官，管理着一支民用团队，该团队为四个区域约克郡部队提供法医成像服务，并应要求在全国各地其他部队提供。他们与一系列媒体合作，包括视频，摄影，CCTV和2D/3D建模。
尽管举止适当，但Dan还是使用CCTV与3D激光扫描技术开发了一些革命性的法医技术应用。3D激光扫描有时也称为LiDAR，用于生产高度准确的3D“点云”（扫描），用于3D数字建模。
Figuring out how to solve complex evidentiary challenges in this way has won Dan a national innovation award from Leica Geosystems, a leading provider of 3D laser scanning equipment to UK police forces. It has also taken him to a conference in Las Vegas, where he presented his techniques to police officers from across the USA, who said they had “never seen anything like it”.
Laser scanning and ‘image stitching’
3D laser scanning’s ability to create highly accurate digital models of crime scenes is impressive enough. The non-intrusive technique helps CSIs to prevent contamination of the scene, and the results can help investigating officers, lawyers and jury members to ‘walk through’ the scene at any time, even years later, and verify details such as distances and lines of sight.
更令人印象深刻的是，可以将3D扫描与现场或附近拍摄的照片或视频剧照一起“缝制”。有时，这种缝制纯粹是为了在模型中添加兴趣点/现实主义或帮助观众更好地了解他们的看法。
However, when Dan’s colleagues brought him a head-scratching series of live cases where CCTV footage was available but lacking in evidentiary significance, he was forced to push stitching further than it had ever gone before.
First, he was asked if a quick burst of outdoor CCTV footage could be used to ascertain the speed of a pedestrian. Second, could CCTV footage be helped to corroborate or eliminate suspects in robbery cases? And third, based on the known locations of two bullet holes, was it possible to establish where the shooter had been standing and whether the gun had been aimed at police officers?
Initially, Dan and his colleague had no idea. But with his expert knowledge of his Leica 3D scanners and software, and his ability to ‘stitch’ digital images into his 3D scans, Dan said he’d give it a go.
A string of convictions
Dan’s efforts paid off. He developed three techniques involving 3D ‘point cloud’ (scan data) models blended with stills taken from CCTV footage: Suspect Height Analysis, Speed Analysis, and Bullet Trajectory Analysis. They’ve since been relied upon in court many times by many police forces: to the best of Dan’s knowledge, the evidence they produce has never been shown to be flawed, never been successfully argued by the defence, and never failed to contribute towards a conviction.
在过去的8年里,甲型肝炎丹的三个方法e been used by the growing Crime Surveying team at West Yorkshire Police, helping to secure convictions for serious crimes including murder, armed robbery, death by dangerous driving, arson with intent to endanger life, and the attempted murder of a police officer. And while the team doesn’t get a mention in the press when cases result in convictions, ‘crucial CCTV evidence’ is often noted!
毫不奇怪，有关West Yorkshire Police’swork in this area continues to spread and he gets plenty of requests from the four Yorkshire forces and beyond. A second force, Essex, has also recently begun using the speed analysis technique.
决定何时应用该技术
根据丹,不是每个案例适用于these methods. “The first thing we need to do in every instance is review what we have in terms of footage – can we see what we need to see? Is the frame rate high enough? Has the scene radically changed since the incident took place? Can this footage stand up in court? We only start the process when we think we’ve got a realistic chance of producing some standalone or corroborating evidence.
“The process can be laborious – taking anything from a couple of days to a few weeks to visit the crime scene, assess the footage, stitch the imagery together, analyse the findings and write up our reports. But it’s worth it: we’re able to offer the court such small margins of error that the chances of our evidence being wrong are very small – sometimes millions to one.”
Dan and his team have regularly accompanied their evidence to court. The 3D models deliver vital evidence, but they also help witnesses to give their testimony in a way which is easier for the court to understand (“I was standing here,” “I couldn’t see that from where I was”). And because it’s possible to rotate and zoom in and out of the models, they help judge and jurors to weigh up that testimony and to orient key individuals and pieces of evidence within the scene.
Innovation award
The laser scanning equipment Dan used to develop his techniques is from Leica Geosystems, a global optics company with its UK headquarters in Milton Keynes. He used a Leica laser scanner with Leica Cyclone 3D point cloud processing software. Today, the team has three different types of Leica laser scanner to call upon.
Mike Skicko is the lead for forensics at Leica Geosystems in the UK and has known Dan for some years. “We’ve been really impressed by the way Dan has taken existing technologies and found clever new forensic applications for them.”
“曾经是不可能的，或者充其量只花了很长时间，测量磁带和大量数学 - 具有人为错误的很大风险和大量容忍度的风险 - 现在可以非常快速，准确地完成。这就是为什么在第一次子弹轨迹分析之后，西约克郡警察在2014年获得了莱卡（Leica）的HDS（高清测量）奖。”
真实情况
Suspect Height Analysis
Until Dan tried his height analysis technique, his force had been paying an external consultant to use traditional photogrammetry to analyse suspects’ images from the scene. This was slow, delivered only 2D models, and had a 2” margin of error either way – which is large, because a lot of the population would fall in the height grouping of, say, 5’7” to 5’11” tall.
But by stitching 3D laser scans and CCTV together, and adding some basic trigonometry techniques, Dan found he could assess a suspect’s height much more accurately, slashing the margin of error to just 0.6” either way. In that example, therefore, this would narrow the pool of suspects to those measuring between 5.84” and 5.96”. This can help the police to dismiss or corroborate evidence about suspects.
The technique proved invaluable following a series of five armed robberies at restaurants and bars in Leeds. Three suspects were repeatedly caught on CCTV, but their faces were covered and they left no footprints or fingerprints. Mobile phone data put the suspects in the vicinity, but was not sufficiently conclusive.
Dan’s technique found that the three suspects must measure 5’4”, 5’11” and 6’5” (he nicknamed them Small, Medium and Large). He found that the suspects’ heights were the same at every crime, indicating that the same men were involved in each. Using percentile height charts for the male British population, Dan found that the chances of these suspects not being the same three individuals together on the footage was millions to one. The suspects had originally refused to be measured, but when Dan’s evidence led to arrests the suspects could be measured in custody. They were found to be very close to the heights Dan had calculated.
Speed Analysis
Dan’s first use of 3D laser scanning and CCTV images to calculate speed came after the high-profile killing of a church organist in Sheffield on Christmas Eve. The suspects claimed they had simply been walking behind the man, but that when they overtook him a scuffle had ensued, and that the man had accidentally been killed. And there was limited evidence to the contrary.
但是，丹获得了夜间闭路电视录像，该录像片段首先显示了受害者，然后犯罪嫌疑人沿着道路行走。他扫描了该地区，并计算了受害者和嫌疑人的确切步行速度。他发现，嫌疑人的行走比风琴师更慢，并且在没有显着速度的情况下永远无法赶上。犯罪嫌疑人必须故意从3mph到14英里 /小时（换句话说，从缓慢的步行到冲刺），以攻击并遭受恶性击败风琴师并抓住他的公文包。这证明了他们的意图，一名嫌疑人因谋杀罪至少被判处25年徒刑，而另一名嫌疑人因杀人杀而被判处九年。(BBC News)
The same technique can be used to determine vehicle speed. In one case, a car driver denied speeding after hitting and killing a pedestrian in Rotherham. There were no skid marks on the road and no witnesses. However, CCTV footage from the exterior of a nearby house showed a partial view of the road. It was enough for Dan’s team to prove speeding: the driver was jailed for eight years and banned from driving for 14 years.(BBC News)
子弹轨迹分析
In the 2011 Birmingham riots, 41 masked and hooded men set fire to a pub in order to lure police to the scene. Shots were fired, but the suspects claimed they had only been shooting into the air. West Midlands Police had heard about Dan’s department’s work, and asked what they could do with two bullet holes: a round had passed through a second-floor window before embedding in a wall.
最初，丹不确定他们能提供帮助。但是，他与同事一起调查了现场 - 只是发现窗户已经被替换。但是，通过扫描房间并在受损窗户的CSI照片中缝制，他重新创建了窗户孔的确切位置。将其与墙壁上的孔连接起来，返回射击者唯一可能的射击位置，一个1m2的地面 - CCTV镜头确定了一个在那个确切时间站在那里的嫌疑人。
丹然后缝合四参加警车我nto the scene and mapped the entire trajectory of the bullet, proving that it had missed a police officer’s head by just three feet. A ballistics expert said that a trajectory of just 1.2° lower would have resulted in a head-shot to the police officer, and that the shooter’s 1.2° ‘error’ could be accounted for simply by the pulling of the trigger. The court accepted intent to kill.
This proof of concept took Dan’s team six weeks to figure out, and then six weeks to present in court. It resulted in a conviction for attempted murder, among other things, and the shooter was sentenced to 29 years in prison.(BBC News)
What is 3D laser scanning/LiDAR?
LiDAR stands for light detection and ranging, and it’s carried out using a 3D laser scanner – a bit like a camera – which fires out thousands of laser pulses per second in every direction. When they hit surrounding objects, these pulses reflect back and are recorded by the scanner. The delay between sending and receiving allows the scanner to calculate spatial distances to a very high degree of accuracy.
The ‘point cloud data’ generated by the scanner is put through specialist software which then builds a sophisticated digital 3D model of the scene. This offers permanent, high-precision measurements between all of the objects in the scene – which might include a body, a weapon and a doorway, for example. You can even ‘stand’ inside different parts of the 3D model, thus recreating the crime scene at any time, even years later.
Laser scanning saves a huge amount of time, is more than accurate enough to be relied upon in court, and helps prevent crime-scene contamination. For example, you can put a scanner in a doorway and get all your measurements done before you even set foot in the scene.
