Footwear Evidence

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Lesson 3

• Footwear Evidence

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Footprint the trace everyone left
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Footprint the trace everyone left
Source SKH Lam Woo Memorial Sec. School CSI
Case study (2010)
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Footprint the trace everyone left

• YouTube - Leave No Trace - Footprint Evidence
• (http//www.youtube.com/watch?vqlmZZoLGMpM)

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Important features in footprints
important features significance
e.g. Size of footprint Real size of suspects foot ? approximate height of suspect
1 Shape of footprint Sex of the suspect
2 Type and brand (e.g. hiking shoes) Suspects hobbies
3 Depth of footprint on soft material Weight of suspect
4 wear patterns and specific damages Walking habit of suspect
5 trace materials, such as soil, tar, rocks, and paint Places where suspect had visited
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Discovery of footprint by oblique light
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Taking photo of a footwear impression
a2
a1
Case 1
Case 2
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Taking photo of a footwear impression
Correct
Incorrect
The ratio in case 2 is very close to that of
the real shoe while a significant discrepancy
appears in case 1. Case 1 is a wrong method
since any angle other than 90o in taking the
photo will cause distortion of the image. The
size and shape of the shoe may be misinterpreted.
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Examples of 2D and 3D footprints
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Methods used in collecting footwear impressions

• (1) Electrostatic lifting

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Methods used in collecting footwear impressions

• (1) Electrostatic lifting

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Methods used in collecting footwear impressions

• (2) Casting

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Individual characteristics on a footprint

Figure 1
Figure 2
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Number of Characteristics Probability of Occurrence p
1 1 / 80
2 1 / 3 200
3 1 / 85 333
Probability
Number of Characteristics Probability of Occurrence
1 1 / 10 000
2 1 / 50 000 000
3 1 / 166 666 666 667
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Individual characteristics on a footprint

• It is highly unlikely that two different shoes
  would share all the same identifying
  characteristics. If 2 -3 characteristics share
  between the footprint in a crime scene and the
  suspects shoe, it would be a strong evidence
  that the suspect had been to the crime scene.
  Otherwise, this may eliminate the suspect as
  having been there.

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• Want to knows more ?
• In probability theory, to say that two
  events are independent means that the
  occurrence of one event makes it neither more
  nor less probable that the other occurs.
• For example
• The event of getting a 6 the first time a die
  is rolled and the event of getting a 6 the second
  time are independent.
• If two cards are drawn with replacement from
  a deck of cards, the event of drawing a red card
  on the first trial and that of drawing a red card
  on the second trial are independent.

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Measurement of human gait at crime scene

• Sex ,
• height of the thief ,
• direction of movement,
• walking or running,
• special walking habit,
• carrying load or not,
• drunken or under medication (if the gait shows
  instability)

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Measurement of human gait at crime scene
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Measurement of human gait at crime scene
(1) If footprints are deep and the pace is long,
rapid movement is apparent. Long strides and deep
prints with toe prints deeper than heel prints
indicate running (A) (2) Prints that are deep,
short, and widely spaced, with signs of scuffing
or shuffling indicate the person is carrying a
heavy load (B). (3) Persons walking backward
(C) have a short, irregular stride. The prints
have an unnaturally deep toe, and soil is
displaced in the direction of movement. (4) To
determine the sex (D,E), we should study the size
and position of the footprints. Women tend to be
pigeon-toed, while men walk with their feet
straight ahead or pointed slightly to the
outside. Prints left by women are usually smaller
and the stride is usually shorter than prints
left by men.