西约克郡警察’s 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”。

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.

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.

一系列信念

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’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.

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.”

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是英国Leica Geosystems取证的领导者，并且已经认识DAN已有多年了。“ Dan采用现有技术并为他们找到了巧妙的新法医应用程序的方式给我们留下了深刻的印象。”

“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

Suspect Height Analysis

在丹尝试了他的高度分析技术之前，他的部队一直在支付外部顾问来使用传统的摄影测量法来分析现场嫌疑人的图像。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.

但是，通过将3D激光扫描和CCTV缝合在一起，并添加一些基本的三角学技术，Dan发现他可以更准确地评估犯罪嫌疑人的高度，以任何方式将错误余量削减到0.6英寸。因此，在那个示例中，这将使嫌疑人的库缩小到5.84英寸至5.96英寸之间的嫌疑人。这可以帮助警察驳回或证实有关嫌疑犯的证据。

在利兹的餐馆和酒吧进行了一系列五次武装抢劫，这项技术证明是无价的。三名嫌疑人在闭路电视上反复抓获，但他们的脸被遮住了，没有留下足迹或指纹。手机数据将犯罪嫌疑人置于附近，但没有足够的结论。

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.

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.(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)

Bullet Trajectory Analysis

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.

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.

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)

什么是3D激光扫描/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.

激光扫描可以节省大量的时间,是铁道部e 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.