Photogrammetry Flashcards
7 cards from real ACTAR practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Photogrammetry flashcards as text
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?
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.
What is a 'mesh' in the context of photogrammetric output, and how does it differ from a point cloud?
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.
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?
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.
What is the significance of the 'reprojection error' reported by photogrammetry software after bundle adjustment?
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.
For court admissibility, what documentation should accompany photogrammetric measurements from a crash scene?
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.
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)?
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.
In UAV-based photogrammetry for accident reconstruction, what is the purpose of flying a 'crosshatch' or 'double-grid' pattern?
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.