Certified Photogrammetrist Digital Photogrammetry & Point Clouds 2 — Questions and Answers
Question 1: What is the difference between a DSM and a DTM?
- DSM (Digital Surface Model) includes all features above ground; DTM (Digital Terrain Model) represents only bare earth (Correct answer)
- DSM uses LiDAR data while DTM uses photogrammetric data
- DSM is in vector format and DTM is in raster format
- DSM has higher resolution than DTM by definition
Correct answer: DSM (Digital Surface Model) includes all features above ground; DTM (Digital Terrain Model) represents only bare earth
A DSM (Digital Surface Model) represents the top of all surfaces including buildings and vegetation, while a DTM (Digital Terrain Model) represents only the bare-earth terrain after removing above-ground features.
Question 2: What is 'occlusion' in the context of dense image matching?
- Radiometric saturation in bright areas
- Areas not visible in one or more images due to blocking by objects (Correct answer)
- Lens vignetting at image edges
- GPS signal dropout during acquisition
Correct answer: Areas not visible in one or more images due to blocking by objects
Occlusion occurs when objects (buildings, trees) block the line of sight to certain areas in some images, creating gaps or reduced accuracy in the resulting point cloud.
Question 3: In UAV/drone photogrammetry, what overlap configuration is typically recommended for high-quality 3D reconstruction?
- 60% forward overlap and 30% sidelap
- 80–90% forward overlap and 70–80% sidelap (Correct answer)
- 50% forward overlap and 50% sidelap
- 95% forward overlap and 20% sidelap
Correct answer: 80–90% forward overlap and 70–80% sidelap
High-quality UAV 3D reconstruction typically requires 80–90% forward overlap and 70–80% sidelap to ensure every surface area is seen from multiple angles for robust matching.
Question 4: What does the LAS file format store in photogrammetric and LiDAR workflows?
- Raster orthophoto data in tiles
- Standardized binary point cloud data including XYZ coordinates, intensity, classification, and return information (Correct answer)
- Camera calibration parameters
- GPS trajectory data from the aircraft
Correct answer: Standardized binary point cloud data including XYZ coordinates, intensity, classification, and return information
The LAS format is an ASPRS-standardized binary file format for storing point cloud data, including XYZ coordinates, intensity, RGB color, classification codes, GPS time, and return numbers.
Question 5: What is 'noise filtering' in point cloud processing?
- Removing radiometric speckle from raster images
- Eliminating outlier points that do not represent actual surfaces (e.g., flying points) (Correct answer)
- Filtering GPS positions with poor PDOP
- Removing duplicate image exposures from the project
Correct answer: Eliminating outlier points that do not represent actual surfaces (e.g., flying points)
Noise filtering removes outlier or erroneous points—such as isolated flying points far from the main surface—that would degrade the accuracy of derived terrain models.
Question 6: What is 'multi-ray' matching in photogrammetric dense reconstruction?
- Matching image features using only two overlapping images (stereopair)
- Simultaneously using three or more overlapping images to improve matching reliability and accuracy (Correct answer)
- A technique for matching features across different spectral bands
- A method for connecting flight strips using cross-tie points
Correct answer: Simultaneously using three or more overlapping images to improve matching reliability and accuracy
Multi-ray matching uses three or more overlapping images simultaneously to verify and improve the reliability of matched points, reducing errors compared to stereopair-only matching.
Certified Photogrammetrist Digital Photogrammetry & Point Clouds 2
What is the difference between a DSM and a DTM?