Reference-Object Method
Definition
Height estimation by measuring the pixel heights of both the subject and a reference object of known stature in the same frame, then deriving the subject's height by proportion with corrections for position differences.
- Requires
- Subject and object of known height in the same frame
- Output
- Estimated stature by pixel-height proportion
- Correction needed
- Position differences between subject and reference object
- Application
- Height estimation from CCTV footage
Common questions
What error does the reference-object method introduce if the subject and object are not at the same distance from the camera?+
Because CCTV lenses are not orthographic, an object further from or closer to the camera than the subject projects to a disproportionate pixel height, so uncorrected measurements can be biased by several centimetres. Photogrammetric correction for depth and lens distortion is needed to keep the estimate reliable.
What makes a good reference object for this method?+
A fixed, permanently measurable feature in the same plane as the subject's path, such as a door frame or marked floor tile, is preferred over a moveable object, because its true height can be verified on a return scene visit and it removes uncertainty about whether the object itself shifted between the recording and the measurement.
Related terms
- Ground-Plane Calibration
- A physical survey of the scene after the event, measuring reference points on the floor relative to the camera position, allowing the...
- Perspective Distortion Error
- The apparent change in size of objects at different distances from the camera due to projective geometry. A person closer to the...
- Pixel Height Ratio
- The number of pixels occupied by a vertical extent (person, door, markings) in the image. Ratios between subject and reference pixel heights...
- Stature Estimation Uncertainty
- The combined error from pixel localisation, reference height knowledge, positional correction accuracy, and camera model assumptions, expressed as a confidence interval at...
- Vanishing-Point Method
- Using the vertical vanishing point and two horizontal vanishing points to compute the camera's effective viewing geometry, then projecting the person's image...