ACTAR Photogrammetry 5 — Questions and Answers
Question 1: What is the primary advantage of LiDAR (Light Detection and Ranging) over photogrammetry for accident scene documentation, and what is its main limitation?
- LiDAR is cheaper and requires no software; its limitation is lower accuracy
- LiDAR captures accurate 3D data rapidly even in low light and does not depend on surface texture; its main limitation is higher equipment cost compared to camera-based photogrammetry (Correct answer)
- LiDAR produces color images while photogrammetry does not; its limitation is slower data collection
- LiDAR works only indoors; photogrammetry is required outdoors
Correct answer: LiDAR captures accurate 3D data rapidly even in low light and does not depend on surface texture; its main limitation is higher equipment cost compared to camera-based photogrammetry
LiDAR directly measures distances using laser pulses and works in poor lighting without needing surface texture, but the equipment cost is substantially higher than camera-based photogrammetric systems.
Question 2: When preparing a photogrammetric model for courtroom presentation, what is the advantage of using a textured 3D mesh over a raw point cloud?
- Textured meshes are smaller file sizes than point clouds
- Textured meshes appear more photorealistic and intuitive to jurors, making spatial relationships easier to understand (Correct answer)
- Textured meshes contain more measurement data than point clouds
- Point clouds cannot be used in court, making meshes the only legal option
Correct answer: Textured meshes appear more photorealistic and intuitive to jurors, making spatial relationships easier to understand
Applying photographic texture to a mesh creates a visually realistic model that non-technical jurors can interpret more easily than an abstract collection of colored dots.
Question 3: What does the term 'self-calibration' mean in photogrammetry?
- The camera automatically adjusts its focus during image capture
- The photogrammetric software simultaneously estimates camera calibration parameters (focal length, distortion coefficients) during the processing of scene images (Correct answer)
- A self-contained camera that requires no external power
- The process of cleaning the camera sensor in the field
Correct answer: The photogrammetric software simultaneously estimates camera calibration parameters (focal length, distortion coefficients) during the processing of scene images
Self-calibration allows the software to solve for lens parameters alongside scene geometry, which is particularly useful when pre-calibration was not performed before fieldwork.
Question 4: A crash occurred on a curve with significant superelevation. How can a photogrammetric DEM help the reconstructionist?
- The DEM shows the color of road markings at the curve
- The DEM provides precise cross-slope and grade measurements that can be used as inputs to vehicle dynamics and critical-speed calculations (Correct answer)
- The DEM identifies the driver's sight lines from the cockpit
- The DEM records vehicle speed at the time of the crash
Correct answer: The DEM provides precise cross-slope and grade measurements that can be used as inputs to vehicle dynamics and critical-speed calculations
Extracting cross-slope (superelevation) and longitudinal grade from a DEM provides accurate roadway geometry inputs needed for friction-based speed and stability calculations.
Question 5: What is 'georeferencing' in the context of UAV photogrammetry, and how is it typically achieved?
- Translating the model description into multiple languages
- Associating the photogrammetric model with a real-world coordinate system using GPS-tagged images, ground control points, or a combination of both (Correct answer)
- Adjusting image colors to match a reference photograph
- Identifying the geographic region where the crash occurred for filing purposes
Correct answer: Associating the photogrammetric model with a real-world coordinate system using GPS-tagged images, ground control points, or a combination of both
Georeferencing links the model to a real-world coordinate system, enabling it to be overlaid on maps and allowing absolute position measurements using GCPs or RTK/PPK GPS.
Question 6: When a photogrammetric model of a crash scene is completed months after the original investigation, which type of evidence is most critical for validating the model's accuracy?
- Eyewitness recollections of scene conditions
- Field-measured check points (independent survey coordinates not used in processing) compared against model-extracted coordinates (Correct answer)
- The reconstructionist's professional experience and reputation
- A second photogrammetric model created from internet satellite imagery
Correct answer: Field-measured check points (independent survey coordinates not used in processing) compared against model-extracted coordinates
Independent check points—survey coordinates withheld from processing—provide an objective accuracy assessment by comparing their known positions to model-extracted positions.
Question 7: In photogrammetric accident reconstruction, what does 'absolute accuracy' mean versus 'relative accuracy'?
- Absolute accuracy refers to the number of images captured; relative accuracy refers to how many are usable
- Absolute accuracy describes how closely model coordinates match true real-world coordinates; relative accuracy describes how correctly internal distances and shapes are reproduced within the model regardless of its position in the world (Correct answer)
- Absolute accuracy applies only to GPS measurements; relative accuracy applies only to total station measurements
- Relative accuracy is always higher than absolute accuracy in any photogrammetric system
Correct answer: Absolute accuracy describes how closely model coordinates match true real-world coordinates; relative accuracy describes how correctly internal distances and shapes are reproduced within the model regardless of its position in the world
A model can have excellent relative accuracy (correctly proportioned internally) but poor absolute accuracy (incorrectly placed in the world) if georeferencing is weak or absent.
ACTAR Photogrammetry 5
What is the primary advantage of LiDAR (Light Detection and Ranging) over photogrammetry for accident scene documentation, and what is its main limitation?