Certified Photogrammetrist Digital Photogrammetry & Point Clouds 1 — Questions and Answers
Question 1: What algorithm is most commonly used for automatic image matching to generate dense point clouds?
- Semi-Global Matching (SGM) or Multi-View Stereo (MVS) (Correct answer)
- RANSAC feature rejection only
- Kriging interpolation
- Inverse distance weighting
Correct answer: Semi-Global Matching (SGM) or Multi-View Stereo (MVS)
Semi-Global Matching (SGM) and Multi-View Stereo (MVS) are the dominant algorithms for dense image matching that produce high-density point clouds from overlapping photographs.
Question 2: What is 'ground sample distance' (GSD) in digital photogrammetry?
- The distance between GPS ground control points
- The real-world size represented by one pixel in the image (Correct answer)
- The spacing between flight lines
- The minimum detectable elevation difference
Correct answer: The real-world size represented by one pixel in the image
GSD (Ground Sample Distance) is the real-world dimension represented by a single pixel in the image, determining the spatial resolution of the final mapping products.
Question 3: Which type of point cloud classification separates ground returns from vegetation and structures?
- Radiometric classification
- LAS class 2 (ground) filtering using algorithms like Progressive TIN or CSF (Correct answer)
- K-means spectral clustering
- Principal component analysis
Correct answer: LAS class 2 (ground) filtering using algorithms like Progressive TIN or CSF
Ground classification assigns LAS class 2 to bare-earth returns using algorithms like Progressive Triangulated Irregular Network (PTIN) or Cloth Simulation Filter (CSF) that distinguish ground from above-ground features.
Question 4: What is a Structure from Motion (SfM) workflow in photogrammetry?
- A process that reconstructs 3D structure and camera positions simultaneously from unordered image sets (Correct answer)
- A method for generating animated fly-throughs
- A radiometric correction technique for haze removal
- A film scanning workflow for analog photography
Correct answer: A process that reconstructs 3D structure and camera positions simultaneously from unordered image sets
Structure from Motion (SfM) simultaneously estimates 3D scene structure and camera position/orientation from a set of overlapping images, regardless of their initial order or known camera positions.
Question 5: In a LiDAR or photogrammetric point cloud, what does 'point density' measure?
- The color saturation of each point
- The number of points per unit area (e.g., points per square meter) (Correct answer)
- The vertical accuracy of individual points
- The compression ratio of the LAS file
Correct answer: The number of points per unit area (e.g., points per square meter)
Point density is the number of points per unit area (typically points/m²), which determines the level of surface detail and the accuracy of derived products like DEMs.
Question 6: What is 'image radiometric normalization' used for in digital photogrammetry?
- Adjusting image brightness and color consistency across a mosaic to reduce seam visibility (Correct answer)
- Converting image coordinates to geographic coordinates
- Increasing the focal length of the camera digitally
- Removing geometric distortion caused by lens aberrations
Correct answer: Adjusting image brightness and color consistency across a mosaic to reduce seam visibility
Radiometric normalization adjusts brightness, contrast, and color across overlapping images to achieve visual consistency in the final orthophoto mosaic, reducing visible seams.
Certified Photogrammetrist Digital Photogrammetry & Point Clouds 1
What algorithm is most commonly used for automatic image matching to generate dense point clouds?