西约克郡警察局imaging team has achieved a string of criminal convictions thanks to their innovative blending of 3D laser scanning and CCTV technologies.

Dan Sharp was working as an electrical surveyor when he saw that West Yorkshire Police were advertising for a crime scene surveyor, and his interest was piqued. He applied, got the job, and has never looked back. Today he’s senior imaging officer in Wakefield, managing a civilian team which provides forensic imaging services for the four regional Yorkshire forces and, on request, other forces around the country. They work with a range of media including video, photography, CCTV and 2D/3D modelling.

Despite his modest demeanour, Dan has developed some revolutionary forensic technology applications using CCTV in combination with 3D laser scanning technology. 3D laser scanning is also sometimes known as LiDAR and is used to produce highly accurate 3D ‘point clouds’ (scans) for 3D digital modelling.

弄清楚如何以这种方式解决复杂的证据挑战，赢得了丹（Dan）的国家创新奖，莱卡·吉尔斯特（Leica Geosystems）是英国警察部队的3D激光扫描设备的领先提供商。它还将他带到了拉斯维加斯的一次会议，在那里他向来自美国各地的警察提出了技术，他们说他们“从未见过类似的东西”。

激光扫描和“图像缝制”

3D激光扫描能够创建高度准确的犯罪场景数字模型的能力，这是足够的。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.

Even more impressively, 3D scans can be ‘stitched’ together with photos or video stills taken at or near the scene. Sometimes this stitching is done purely to add points of interest/realism to the model or to help the viewer better understand what they’re looking at.

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.

Over the past eight years, Dan’s three methods have 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!

Unsurprisingly, news of西约克郡警察局work 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.

Deciding when to apply the technique

根据Dan的说法，并非每种情况都适合这些方法。“在每种情况下，我们需要做的第一件事是查看我们在录像方面所拥有的东西 - 我们可以看到我们需要看到的东西吗？帧速率足够高吗？自事件发生以来，现场发生了根本变化吗？这个录像可以在法庭上站立吗？我们只有在认为我们有现实的机会来产生一些独立或证实证据的机会时才开始流程。

“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.”

丹和他的团队定期陪同他们的证据向法庭。3D模型提供了重要的证据，但它们也帮助证人以更容易理解的方式做出证词（“我站在这里”，“我看不到我的位置”）。而且，由于可以旋转和放大模型，因此它们可以帮助判断和陪审员权衡该证词，并在现场的关键人物和证据方面定义。

创新奖

DAN用于开发技术的激光扫描设备来自Leica Geosystems，这是一家全球光学公司，其英国总部位于Milton Keynes。他使用了Leica激光扫描仪与Leica Cyclone 3D点云处理软件。如今，该团队有三种不同类型的Leica Laser扫描仪需要呼吁。

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.”

“What might once have been impossible, or at best have taken ages to piece together using 2D photos, measuring tapes and a lot of maths – with a big risk of human error and significant margins of tolerance – can now be done very quickly and accurately. It’s why West Yorkshire Police won Leica’s HDS (high definition surveying) Award for Innovation in 2014, following the first bullet trajectory analysis.”

Real cases

可疑身高分析

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.

丹的技术发现,三名嫌疑人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.

However, Dan was provided with night-time CCTV footage which briefly showed first the victim and then the suspects walking along the road. He scanned the area and calculated the exact walking speed of the victim and suspects. He found that the suspects had been walking more slowly than the organist and could never have caught up without a significant change of pace. The suspects must have deliberately accelerated from 3mph to 14mph – in other words, from a slow walk to a sprint – to attack and viciously beat the organist and seize his briefcase. This proved their intent, and one suspect was sentenced to a minimum of 25 years for murder while the other received nine years for manslaughter.（英国广播公司的新闻）

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.（英国广播公司的新闻）

Bullet Trajectory Analysis

在2011年的伯明翰暴动中，有41名蒙面和连帽衫的男子纵火向酒吧着火，以吸引警察到现场。开枪射击，但犯罪嫌疑人声称他们只是在空中射击。西米德兰兹警察局听说了丹的部门的工作，并问他们可以用两个子弹孔做什么：一轮在嵌入墙壁之前穿过二楼的窗户。

Initially, Dan wasn’t sure they’d be able to help. However, he went to survey the scene with a colleague – only to discover that the window had already been replaced. But by scanning the room and stitching in a CSI photo of the damaged window, he recreated the exact position of the window’s hole. Connecting it with the hole in the wall led back to the shooter’s only possible firing position, a 1m2 patch of ground – and CCTV footage identified a single suspect who had been standing there at that exact time.

Dan then stitched the four attending police vans into 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.

该概念证明花了丹的团队六个星期才能弄清楚，然后在法庭上征求六个星期。这导致因未遂谋杀而定罪，除其他外，射手被判处29年徒刑。（英国广播公司的新闻）

What is 3D laser scanning/LiDAR?

激光雷达代表光探测和测距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.

扫描仪生成的“点云数据”是通过专业软件进行的，然后构建了现场的复杂数字3D模型。这提供了场景中所有对象之间的永久高精度测量，例如，可能包括车身，武器和门口。您甚至可以在3D模型的不同部分内“站在”，从而随时甚至在几年后重新创建犯罪现场。

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.