Digital Photogrammetry & Point Clouds Flashcards

What algorithm is most commonly used for automatic image matching to generate dense point clouds?

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.

What is 'ground sample distance' (GSD) in digital photogrammetry?

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.

Which type of point cloud classification separates ground returns from vegetation and structures?

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.

What is a Structure from Motion (SfM) workflow in photogrammetry?

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.

In a LiDAR or photogrammetric point cloud, what does 'point density' measure?

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.

What is 'image radiometric normalization' used for in digital photogrammetry?

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.