Certified Photogrammetrist Camera Calibration & Sensor Geometry 2 — Questions and Answers
Question 1: What causes decentering (tangential) lens distortion?
- Curvature mismatch of the lens elements
- Misalignment of optical elements from the optical axis within the lens system (Correct answer)
- Temperature variations during flight operations
- Atmospheric refraction of light rays
Correct answer: Misalignment of optical elements from the optical axis within the lens system
Decentering distortion is caused by the physical misalignment or decentering of lens elements from the optical axis, producing an asymmetric distortion pattern.
Question 2: What is the purpose of a reseau grid (reseau plate) in analog aerial cameras?
- To filter ultraviolet radiation from sunlight
- To provide reference marks for detecting and correcting film deformation (Correct answer)
- To improve image sharpness at the corners of the frame
- To record the exact time of each exposure
Correct answer: To provide reference marks for detecting and correcting film deformation
A reseau grid contains precisely positioned crosses or dots imaged onto the film, allowing detection and correction of film deformation that may occur during processing or scanning.
Question 3: In digital aerial cameras, what component replaces the photographic film at the focal plane?
- The lens system
- The shutter mechanism
- The CCD or CMOS image sensor (Correct answer)
- The viewfinder assembly
Correct answer: The CCD or CMOS image sensor
In digital cameras, the CCD or CMOS sensor array is positioned at the focal plane to capture light, directly replacing the role of photographic film as the image recording medium.
Question 4: In digital photogrammetry, pixel size of the sensor is most directly related to:
- The focal length of the camera lens
- The ground sample distance (GSD) (Correct answer)
- The shutter speed setting
- The lens radial distortion magnitude
Correct answer: The ground sample distance (GSD)
Pixel size directly determines the ground sample distance (GSD) for a given flying height and focal length; GSD defines the smallest ground detail that can be resolved.
Question 5: Which camera calibration parameter corrects for non-orthogonality (shear) between the x and y axes of the image coordinate system?
- Radial distortion coefficient K1
- Principal point offset
- Affinity (shear) parameter (Correct answer)
- Focal length correction
Correct answer: Affinity (shear) parameter
The affinity or shear parameter corrects for non-orthogonality between the image coordinate axes, which can result from imperfect sensor geometry or digitizer scanning.
Question 6: The Modulation Transfer Function (MTF) in optical and sensor systems is used to measure:
- The accuracy of onboard GPS position measurements
- The ability of the system to resolve and transfer spatial frequency detail (Correct answer)
- The radial lens distortion profile
- The sensor dark current noise level
Correct answer: The ability of the system to resolve and transfer spatial frequency detail
MTF quantifies how well an optical system transfers contrast from the scene to the image at different spatial frequencies, characterizing the overall resolving power and sharpness of the system.
Question 7: A camera with a 150 mm focal length is flown at 3,000 m above ground level. What is the approximate nominal image scale?
- 1:200
- 1:2,000
- 1:20,000 (Correct answer)
- 1:200,000
Correct answer: 1:20,000
Image scale = f/H = 0.150 m / 3,000 m = 1/20,000, giving a scale of 1:20,000.
Certified Photogrammetrist Camera Calibration & Sensor Geometry 2
What causes decentering (tangential) lens distortion?