ACTAR Photogrammetry 4 — Questions and Answers
Question 1: Which of the following best describes 'orthorectification' in photogrammetric products?
- The process of straightening bent control point rods
- Correcting an image to remove perspective distortion and terrain relief displacement so it has a uniform scale throughout (Correct answer)
- Adjusting image brightness and contrast for printing
- Rotating point clouds to align with magnetic north
Correct answer: Correcting an image to remove perspective distortion and terrain relief displacement so it has a uniform scale throughout
Orthorectification corrects perspective distortion and displacement caused by terrain elevation changes, producing an orthophoto where all pixels have the same ground scale.
Question 2: A reconstructionist needs to determine the pre-impact position of a vehicle using photogrammetric analysis of surveillance video. What unique challenge does video photogrammetry present compared to still photography?
- Video cameras cannot be calibrated
- Video frames have lower resolution and may have rolling shutter distortion, complicating accurate 3D reconstruction (Correct answer)
- Video photogrammetry always requires a UAV
- Surveillance footage is never admissible as photogrammetric evidence
Correct answer: Video frames have lower resolution and may have rolling shutter distortion, complicating accurate 3D reconstruction
Video frames are typically lower resolution than still photos and CMOS sensors in cameras often exhibit rolling shutter artifacts that cause geometric distortions requiring correction.
Question 3: What is a 'digital elevation model' (DEM) in the context of photogrammetric outputs, and why is it useful in accident reconstruction?
- A 3D wire-frame model of a vehicle
- A raster dataset representing the height of the ground surface at each pixel location, useful for analyzing road grade and superelevation (Correct answer)
- A database of driver elevation (height) measurements
- A calibration file specifying camera elevation during image capture
Correct answer: A raster dataset representing the height of the ground surface at each pixel location, useful for analyzing road grade and superelevation
A DEM captures terrain height information across the scene, enabling reconstructionists to measure road grades, superelevation, and drainage patterns relevant to vehicle dynamics.
Question 4: When using photogrammetry to document tire marks, why is it important to photograph them before any rain or road cleaning occurs?
- Rain improves contrast and makes marks easier to photograph
- Tire marks are permanent and unaffected by weather
- Moisture and cleaning can obliterate or alter the physical evidence, making accurate photogrammetric documentation impossible afterward (Correct answer)
- Road crews are required to photograph marks before cleaning
Correct answer: Moisture and cleaning can obliterate or alter the physical evidence, making accurate photogrammetric documentation impossible afterward
Tire marks are transient physical evidence that can be washed away or degraded quickly; photogrammetric documentation must occur before any environmental or human alteration.
Question 5: What is the purpose of the 'scale bar' in close-range photogrammetry when GPS or total station control points are unavailable?
- To measure the photographer's height for perspective correction
- To provide a known physical length that allows the software to determine the real-world scale of the model (Correct answer)
- To calibrate lens aperture settings
- To define the boundary of the area of interest
Correct answer: To provide a known physical length that allows the software to determine the real-world scale of the model
A scale bar with a precisely known length placed in the scene allows photogrammetry software to convert model units to real-world measurements when GPS control is not used.
Question 6: In photogrammetry, what is the 'bundle adjustment' step and why is it critical?
- Physically bundling and organizing the camera equipment after fieldwork
- A simultaneous mathematical optimization that refines both camera positions/orientations and 3D point locations to minimize overall reprojection error (Correct answer)
- Compressing image files into a bundle for archiving
- Adjusting the number of control points used in the model
Correct answer: A simultaneous mathematical optimization that refines both camera positions/orientations and 3D point locations to minimize overall reprojection error
Bundle adjustment simultaneously optimizes all camera parameters and 3D point positions, distributing errors across the entire dataset to achieve the most accurate possible solution.
Question 7: An accident reconstructionist is asked to extract measurements from photographs taken by a first responder who did not use photogrammetric protocols. Which technique might still allow some metric information to be extracted?
- No measurements can ever be extracted from non-photogrammetric images
- Single-image photogrammetry using known object dimensions (like lane width or vehicle size) as references to establish scale (Correct answer)
- The images must be re-taken from the same positions using a calibrated camera
- Measurements can only be obtained by guessing based on the scene
Correct answer: Single-image photogrammetry using known object dimensions (like lane width or vehicle size) as references to establish scale
Single-image or reverse photogrammetry can extract limited measurements from non-compliant images by using independently known dimensions (reference objects) to establish scale and perspective.
ACTAR Photogrammetry 4
Which of the following best describes 'orthorectification' in photogrammetric products?