ACTAR Photogrammetry 3 — Questions and Answers
Question 1: When using a total station to survey ground control points for photogrammetric processing, what coordinate system is typically used to ensure compatibility with mapping products?
- Polar coordinates relative to the total station
- A local arbitrary coordinate system with no real-world reference
- A georeferenced coordinate system such as State Plane or UTM (Correct answer)
- Pixel coordinates from a reference image
Correct answer: A georeferenced coordinate system such as State Plane or UTM
Georeferenced systems like State Plane or UTM tie the photogrammetric model to real-world coordinates, enabling accurate measurements and map overlays.
Question 2: What is a 'mesh' in the context of photogrammetric output, and how does it differ from a point cloud?
- A mesh is a type of camera calibration file, while a point cloud is the final deliverable
- A mesh is a continuous surface created by connecting point cloud vertices with triangular polygons, whereas a point cloud is a collection of discrete 3D points (Correct answer)
- A mesh refers to the GPS network used during data collection
- A mesh is a lower-resolution version of a point cloud
Correct answer: A mesh is a continuous surface created by connecting point cloud vertices with triangular polygons, whereas a point cloud is a collection of discrete 3D points
A mesh connects point cloud vertices into a continuous triangulated surface, enabling watertight 3D models suitable for measurements and visual presentations.
Question 3: An accident reconstructionist is using photogrammetry to measure skid marks. The marks are partially in shadow. What is the best approach to ensure accurate documentation?
- Document only the lit portions and estimate the shadowed sections
- Photograph only during midday when shadows are minimized
- Use additional artificial lighting or capture images at multiple times of day to reduce shadows (Correct answer)
- Increase the ISO setting to maximum to compensate
Correct answer: Use additional artificial lighting or capture images at multiple times of day to reduce shadows
Supplemental lighting or multi-time-of-day capture ensures shadow areas are properly exposed and visible, preventing gaps in the photogrammetric reconstruction.
Question 4: What is the significance of the 'reprojection error' reported by photogrammetry software after bundle adjustment?
- It measures the distance between the camera and the nearest control point
- It quantifies how accurately the software's computed 3D points re-project back onto the original images, indicating model quality (Correct answer)
- It calculates the lens distortion coefficient
- It reports the altitude error of the UAV during flight
Correct answer: It quantifies how accurately the software's computed 3D points re-project back onto the original images, indicating model quality
Low reprojection error (typically <1 pixel) indicates that the 3D model and camera positions are consistent with the original images, signifying high accuracy.
Question 5: For court admissibility, what documentation should accompany photogrammetric measurements from a crash scene?
- Only the final 3D model file
- Camera make and model only
- The original images, calibration data, control point survey records, software used, and methodology description (Correct answer)
- A signed statement from a witness confirming the scene layout
Correct answer: The original images, calibration data, control point survey records, software used, and methodology description
Comprehensive documentation including raw images, calibration parameters, survey records, and methodology supports the foundation for admissibility and allows independent verification.
Question 6: What effect does using a wide-angle lens (e.g., 16mm focal length) have on photogrammetric data collection compared to a standard lens (e.g., 50mm)?
- Wide-angle lenses eliminate all lens distortion
- Wide-angle lenses cover more area per image but introduce greater radial distortion requiring careful calibration (Correct answer)
- Wide-angle lenses produce sharper images with higher resolution
- Wide-angle lenses are preferred because they require fewer photos
Correct answer: Wide-angle lenses cover more area per image but introduce greater radial distortion requiring careful calibration
Wide-angle lenses capture more scene per image (useful for efficiency) but introduce more radial distortion, which must be precisely modeled in calibration to avoid measurement errors.
Question 7: In UAV-based photogrammetry for accident reconstruction, what is the purpose of flying a 'crosshatch' or 'double-grid' pattern?
- To reduce battery consumption
- To ensure imagery is captured only from north-facing directions
- To provide image overlap in two perpendicular directions, improving 3D reconstruction quality and reducing doming artifacts (Correct answer)
- To comply with FAA regulations requiring two flight paths
Correct answer: To provide image overlap in two perpendicular directions, improving 3D reconstruction quality and reducing doming artifacts
A crosshatch pattern ensures images are captured along two perpendicular axes, providing diverse viewing angles that improve point cloud density and reduce systematic warping.
ACTAR Photogrammetry 3
When using a total station to survey ground control points for photogrammetric processing, what coordinate system is typically used to ensure compatibility with mapping products?