Certified Photogrammetrist Stereo Compilation & 3D Modeling 1 — Questions and Answers
Question 1: What is the 'y-parallax' in a stereopair, and what does excessive y-parallax indicate?
- The vertical image displacement caused by relief; it indicates high terrain
- Residual vertical displacement after relative orientation; it indicates incomplete relative orientation (Correct answer)
- The horizontal parallax difference used to measure elevation
- The parallax introduced by atmospheric refraction
Correct answer: Residual vertical displacement after relative orientation; it indicates incomplete relative orientation
Y-parallax is residual vertical displacement of conjugate points in a stereopair after relative orientation; excessive y-parallax indicates that the relative orientation is incomplete or incorrect.
Question 2: In stereo compilation, what does 'absolute orientation' accomplish?
- It scales, rotates, and positions the stereomodel into a real-world coordinate system using ground control (Correct answer)
- It removes y-parallax from the stereopair
- It adjusts image contrast for better stereoscopic viewing
- It determines the interior orientation of each camera
Correct answer: It scales, rotates, and positions the stereomodel into a real-world coordinate system using ground control
Absolute orientation transforms the relatively oriented stereomodel (which exists in an arbitrary coordinate system) into real-world coordinates using ground control points.
Question 3: What is 'x-parallax' used for in stereo photogrammetry?
- Detecting y-parallax errors in the model
- Measuring elevation differences between points in a stereopair (Correct answer)
- Aligning flight strips in a block adjustment
- Determining the principal distance of the camera
Correct answer: Measuring elevation differences between points in a stereopair
X-parallax (the horizontal displacement between corresponding points in a stereopair) is directly related to the elevation of those points and is the fundamental measurement used for height determination.
Question 4: What is a 'floating mark' in analogue or softcopy stereo compilation?
- A GPS receiver that drifts during flight
- A stereoscopic measuring mark that appears to float at a selected elevation within the stereomodel (Correct answer)
- An unregistered control point that is not tied to the datum
- A tie point that only appears in one photograph
Correct answer: A stereoscopic measuring mark that appears to float at a selected elevation within the stereomodel
A floating mark is a stereoscopic measuring reticle that the operator perceives as resting on the terrain surface when the parallax is correctly set, used to measure or compile terrain features.
Question 5: What are the six elements of exterior orientation for a single photograph?
- Focal length, principal point x, principal point y, radial distortion k1, k2, k3
- X, Y, Z (position of perspective center) and ω, φ, κ (three rotation angles) (Correct answer)
- Scale, rotation, translation (2D), and focal length
- Latitude, longitude, altitude, roll, pitch, yaw (as GPS/IMU output only)
Correct answer: X, Y, Z (position of perspective center) and ω, φ, κ (three rotation angles)
Exterior orientation consists of six parameters: three for the position of the perspective center (X₀, Y₀, Z₀) and three rotation angles (omega ω, phi φ, kappa κ) defining the camera's attitude.
Question 6: What is the 'base-to-height ratio' (B/H ratio) in aerial stereophotogrammetry?
- The ratio of flying height to camera focal length
- The ratio of the air base (distance between exposure stations) to the flying height, affecting depth measurement sensitivity (Correct answer)
- The ratio of forward overlap to sidelap percentage
- The ratio of image scale to map scale
Correct answer: The ratio of the air base (distance between exposure stations) to the flying height, affecting depth measurement sensitivity
The base-to-height ratio (B/H) is the ratio of the air base to the flying height; a larger B/H ratio increases the sensitivity of parallax-based height measurements but also increases relief displacement.
Certified Photogrammetrist Stereo Compilation & 3D Modeling 1
What is the 'y-parallax' in a stereopair, and what does excessive y-parallax indicate?