John Walsh

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John Walsh

• Suspect Height Measurement Using PhotoModeler
  Photogrammetric Software
• Photarc Surveys Limited

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Whats coming up?

• Description of photogrammetry
• Description of PhotoModeler
• A sample project
• The pros and cons

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What is photogrammetry?

• The science (and sometimes the art) of extracting
  measurements from photographic images

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What is it used for?

• Aerial survey
• Architecture
• Engineering
• Forensic work
• and a lot more...

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How does it work?

• Photography is taken with a calibrated camera
• Survey control is measured
• Photogrammetric 3D model is formed
• Data is collected

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Multi-image photogrammetry
Image 3
Image 1
Image 2
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What is PhotoModeler?

• Multi-image, close-range photogrammetry software
• Works with digital or scanned film images
• Allows multi or single-image analysis
• Calibrates cameras

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What do I need?
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How tall is he?
?
We can try...
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guesswork...
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or measurement

• The door method - precision eh?

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Why PhotoModeler?

• PhotoModeler Pro 5 has two useful tools
• Inverse camera function recovers internal
  geometry of an unknown camera
• Surface draw enables feature capture on flat
  surfaces from single images

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Single-image analysis
Local co-ordinate system
Camera position and geometry
Measurement plane through suspect
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Decide if the CCTV image is suitable

• Is the detail clear and not obstructed?
• Are there enough control points?
• Is the subject blurred from movement?
• Is the subject upright?

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Simulation Project

• VGA resolution image simulates CCTV camera
• Scene model created with calibrated camera
• Person of known height used

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Scene model
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Orientate the CCTV image

• 2) Calculate CCTV position and internal geometry
  

1) Digitise the control
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Locate suspect position
3) Create a surface on the floor
4) Place a point at the suspects feet
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Measure suspect height
5) Create a surface through the suspect
6) Place a point on top of the suspects head
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Verify check distances
7) Measure check distance
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Look, no feet!
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Results
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How can we be sure?

• Measure check distances visible in the CCTV
  image
• Measure the suspect from several images
• Measure a person of known height using the same
  CCTV camera

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Pros

• Turns CCTV / bystansder images into measurement
  tools
• Archive photography
• Short site time
• User-friendly

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Cons

• Dependent on image quality
• Dependent on camera position
• Dependent on clear points or lines of perpective
  
• Despte user-friendly software, still needs
  initial training investment

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Thank you for listening