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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?v=qlmZZoL
GMpM )
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important features significance
1
2 e.g.
3
4
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Size of footprint
Shape of footprint
Type and brand
(e.g. hiking shoes)
Depth of footprint on soft material wear patterns and specific damages
Real size of suspect
’ s foot
 approximate height of suspect
Sex of the suspect
Suspect ’ s hobbies
Weight of suspect
Walking habit of suspect trace materials, such as soil, tar, rocks, and paint
Places where suspect had visited
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Taking photo of a footwear impression a
1
Case 1 a
2
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 90 o in taking the photo will cause distortion of the image. The size and shape of the shoe may be misinterpreted.
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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
1
2
3 number of
Probability = total number of ways of possible success outcomes p

2

1
80

80

1
3200 p
3

3

2

1
80

80

80

1
85333
Probability of
Occurrence p
1 / 80
1 / 3 200
1 / 85 333
Probability of
Occurrence Number of
Characteristics
1
2
3
1 / 10 000
1 / 50 000 000
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 suspect’s 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.
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