Diary of a Criminal Investigator-Chapter 259: Technical Analysis

From a professional perspective, footprint identification and analysis primarily targets three elements.

The imprinting object refers to the main subject that creates the footprints, such as a barefoot, a foot wearing a shoe, or a foot wearing a sock.

The imprinted object is the ground or other objects that carry the footprints.

The third critical factor is the force applied, which includes not only the body’s gravity but also the muscular contraction force of the foot.

In the vast majority of cases, the type of footprint encountered is a shoeprint.

It’s rare for suspects to not wear shoes, walking barefoot or only in socks at the crime scene freely.

Footprint identification that Lu Chuan has encountered is also related to shoeprint footprints.

Compared to barefoot prints or socked footprints, shoeprints have a distinct feature as the shape is strictly confined by the shoe’s external form.

For example, a foot size of 40 can fit into size 40 shoes, or even larger sizes like 42 or 43.

This introduces a lot of uncertainty in footprint identification and analysis.

Therefore, the size and length of footprints can be misleading in footprint identification and analysis.

Ordinary crime scene investigations use a fixed formula to estimate based on foot length and height, which inevitably contains errors.

Although Lu Chuan hasn’t yet gone to the scene to see the footprints or footprint models, from the photos alone, he knew that the crime scene investigation by the High-speed Rail New District Criminal Investigation Team had an issue with estimating the suspect’s height. 𝑓𝘳𝑒𝑒𝓌𝘦𝘣𝘯ℴ𝑣𝘦𝑙.𝘤𝑜𝑚

Lu Chuan first analyzed the measurement data of the shoeprints.

By comparing data like full foot length, arch width, ball width, and heel width, one can roughly estimate the footprint owner’s height and weight.

But this only gives a range.

Specifically, one also needs to consider the depth of the footprint left on the ground.

This aspect requires experimentation for verification by comparing it with the shoeprint owner’s weight.

This is actually the simplest part.

People of different weights leave footprints of varying depths on the same ground.

By finding someone with a known weight to step on the spot where the suspect’s shoeprints were left, and comparing the depth of both footprints, the suspect’s weight can be calculated.

However, since Lu Chuan has only seen the photos now, he cannot perform these measurements.

He can only analyze the basic conditions.

The crime scene investigation by the High-speed Rail New District Criminal Investigation Team collected twelve sets of footprints at the scene.

With so many footprints, among them, there are footprints left by the suspect during continuous movement.

In this way, Lu Chuan can analyze the gait characteristics.

First, it’s the stride characteristics.

One can determine the suspect’s walking stride length, stride angle, and stride width.

However, generally speaking, identifying stride characteristics can’t be done with just one or two footprints.

It requires continuous footprints, at least more than four advancing ones, to analyze this aspect.

With continuous footprints, measurement becomes relatively easy.

"Stride length... 45 centimeters."

Generally, the stride length of adult males ranging in height from 1.7 to 1.8 meters is usually between 50 to 80 centimeters.

Of course, crime scene measurements of stride length also have requirements.

The distance from the tip of the back foot to the heel of the front foot, excluding the lengths of both feet, is the stride length.

However, the suspect’s stride length is only 45 centimeters.

This data is significantly below the average stride length for adult males.

On the crime scene, there are three standards for judging stride length. Generally speaking, more than 80 centimeters is a long stride, 71 to 80 centimeters is a medium stride.

Below 70 centimeters is a short stride.

The suspect’s stride length is the shortest of short strides.

However, combined with the previous deduction about the suspect’s height by Lu Chuan, it still makes sense.

"Stride angle... 7.2 degrees outward"

Most people’s stride angle is around 6 degrees, close to 7 degrees suggests the suspect might be slightly pigeon-toed.

"Stride width... 8.1 centimeters, normal."

The measurement and analysis of stride width is actually more complex compared to stride length and stride angle.

Individual stride length and stride angle are not unique.

Finding two people with the same stride length is very easy.

The same goes for stride angle; for most people walking normally, the stride angle is typically around 6.7 degrees.

Even if there are differences, they aren’t particularly significant.

But it’s different with stride width.

In reality, it’s rare to find people with exactly the same stride width.

The crime scene investigation categorized stride width into several categories.

For instance, split strides, tandem strides, overstriding, linear strides, crossed strides, etc.

The suspect’s stride width falls within the range of most normal people.

Finally, Lu Chuan conducted the gait characteristic analysis of the suspect.

Different people’s gait characteristics have unique features.

For example, some people land with their heels first when walking, while others land with the ball of their foot first.

This is the landing feature.

The push-off also has characteristics; some people push off with the ball of the foot, some with the heel, and some with the toes.

Thus, each footprint is actually dynamic.

By examining the variations in pressure at the front, middle, and back of the footprint, many walking features of the suspect can be inferred.

Such as the push-off scratch at the start, which is shown by a deep trace on the ball of the foot in the footprint, causing dirt to shift backward on muddy ground.

Another example is the scooping mark at the start, with the toe pressing down, creating a crescent-shaped depression.

Lu Chuan’s identification and analysis were meticulous, over an hour had passed, and he still hadn’t finished.

Liu Guodong and others did not disturb him.

Like fingerprint identification and analysis, this work requires one to calm down and concentrate.

Of course, compared to fingerprint identification and analysis, footprint identification requires more detailed measurements and data calculations.

According to Lu Chuan’s work experience, conducting footprint identification and analysis is more tiring than fingerprint identification and analysis.

"Sigh..."

Two and a half hours later, almost closing time, Lu Chuan finally exhaled deeply.

"All done?"

Old Bai brought over a cup of tea, which Lu Chuan took and drank down.

"Pretty much, but I still need to see the scene; otherwise, some data might be uncertain."

Lu Chuan looked at Liu Guodong.

The other nodded: "I just called, the scene is still there, you should be able to check it out."

"Shall we go now?"

Old Bai looked at the sky; it would be dark soon.

"Let’s go, the sooner the better."

"Alright, I’ll go with you, Old Yang needs to go home to be with his son, I have nothing to do."

"Master, then we’ll head off."

"Okay, I’ll contact the High-speed Rail New District there, Zhang Qi will send someone to assist you."

An hour later.

The west end of Xinjian Village in the High-speed Rail New District, the cornfield.

At the end of April in Haizhou, corn seedlings have just sprouted, not even as tall as chopsticks.

But they are lush, and the air is fresh.

"Teacher Lu, this is the crime scene."

The person from the High-speed Rail New District was Sun Miao, who collected the footprints during the crime scene investigation.

They knew each other, having attended the city bureau’s fingerprint battle together earlier this year.

However, Sun Miao used to call Lu Chuan by his name, and now he addressed him as Teacher Lu.