Certified Photogrammetrist Camera Calibration & Sensor Geometry — Questions and Answers
Question 1: What is the principal point in camera calibration?
- The point where the optical axis intersects the image plane (Correct answer)
- The geometric center of the image frame
- The midpoint between two fiducial marks
- The projection center of the lens
Correct answer: The point where the optical axis intersects the image plane
The principal point is where the optical axis intersects the image plane, representing the foot of the perpendicular from the perspective center to the image plane.
Question 2: Which distortion model parameter primarily describes symmetric radial lens distortion?
- Affinity coefficient
- K1 radial distortion coefficient (Correct answer)
- Decentering coefficient P1
- Principal point offset
Correct answer: K1 radial distortion coefficient
K1 is the primary radial distortion coefficient that describes symmetric, rotationally symmetric distortion measured from the lens center outward.
Question 3: What does interior orientation define in photogrammetry?
- The position and attitude of the camera in the world coordinate system
- The geometric relationship between the image plane and the perspective center (Correct answer)
- The spatial relationship between overlapping images
- The transformation from pixel to ground coordinates
Correct answer: The geometric relationship between the image plane and the perspective center
Interior orientation defines the geometric relationship between the image coordinate system and the perspective center, including focal length, principal point location, and lens distortion parameters.
Question 4: In aerial photography, fiducial marks are used primarily to:
- Measure ground control points
- Define the image coordinate system (Correct answer)
- Calibrate the onboard GPS receiver
- Determine the aircraft flight altitude
Correct answer: Define the image coordinate system
Fiducial marks are reference points etched into the camera's focal plane that define the image coordinate system and allow transformation from scanner to image coordinates.
Question 5: What is the calibrated focal length (CFL) in photogrammetry?
- The manufacturer's nominal specified focal length
- The distance from the rear nodal point to the film plane
- The focal length value that minimizes the overall effect of radial distortion across the image (Correct answer)
- The focal length measured when focused at infinity
Correct answer: The focal length value that minimizes the overall effect of radial distortion across the image
The calibrated focal length is the value that minimizes the overall effect of radial lens distortion across the image format, and may differ slightly from the manufacturer's specification.
Question 6: Which laboratory calibration method uses precision optical collimators to simulate distant targets at infinity?
- Test field calibration
- Stellar calibration
- Goniometer calibration (Correct answer)
- On-the-job self-calibration
Correct answer: Goniometer calibration
Goniometer calibration uses a precision instrument with collimators to project parallel rays simulating targets at infinity, enabling measurement of angular relationships and lens distortion in a controlled lab environment.
Question 7: The image scale (s) of a vertical aerial photograph with focal length f taken at flying height H above datum is expressed as:
- s = f / H (Correct answer)
- s = H / f
- s = f × H
- s = f + H
Correct answer: s = f / H
Image scale equals focal length divided by flying height above datum (s = f/H); a longer focal length or lower flying height produces a larger (more detailed) scale.
Question 8: 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 9: 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 10: 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 11: 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 12: 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 13: 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 14: 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.
Question 15: What does exterior orientation of a photograph define?
- The internal geometric parameters of the camera
- The position and rotational attitude of the camera at the moment of exposure (Correct answer)
- The calibration parameters of the lens distortion
- The image coordinate system origin
Correct answer: The position and rotational attitude of the camera at the moment of exposure
Exterior orientation defines the six parameters — X, Y, Z position and omega, phi, kappa rotation angles — describing where the camera was and how it was oriented when the photo was taken.
Question 16: What are the three rotation angles used to describe camera orientation in standard photogrammetric convention?
- Roll, pitch, yaw
- Omega, phi, kappa (Correct answer)
- Alpha, beta, gamma
- Azimuth, elevation, bank
Correct answer: Omega, phi, kappa
In photogrammetry, the three rotation angles omega (ω), phi (φ), and kappa (κ) define the rotational orientation of the camera relative to the ground coordinate system.
Question 17: The collinearity condition in photogrammetry states that:
- Control points must be evenly distributed across the image block
- An object point, the camera perspective center, and its image point must lie on a single straight line (Correct answer)
- Flight lines must be parallel for accurate triangulation
- Images in a strip must have uniform scale throughout
Correct answer: An object point, the camera perspective center, and its image point must lie on a single straight line
The collinearity condition states that an object point, the camera's perspective center, and the corresponding image point must lie on the same straight line, which is the fundamental geometric basis of the photogrammetric equations.
Question 18: Which type of aerial camera lens has an angular field of view greater than 100 degrees and is used for small-scale mapping?
- Normal-angle lens (about 60°)
- Wide-angle lens (about 90°)
- Superwide-angle lens (about 120°) (Correct answer)
- Telephoto lens (about 30°)
Correct answer: Superwide-angle lens (about 120°)
Superwide-angle lenses have angular fields of view greater than 100 degrees (typically ~120°), covering large ground areas per photograph, making them suited for small-scale topographic mapping.
Question 19: What is chromatic aberration in photogrammetric camera lenses?
- Distortion caused by temperature-induced changes in the lens
- The failure of a lens to focus different wavelengths of light at the same focal point (Correct answer)
- Motion blurring caused by aircraft vibration during exposure
- Vignetting (light falloff) at the edges of the image frame
Correct answer: The failure of a lens to focus different wavelengths of light at the same focal point
Chromatic aberration occurs when a lens fails to focus all wavelengths of light at exactly the same focal point, causing color fringing or reduced sharpness, particularly visible at high-contrast edges.
Question 20: In photogrammetric sensor calibration, what is meant by 'self-calibration'?
- The camera automatically adjusts its parameters during flight
- Camera interior orientation parameters are solved simultaneously with exterior orientation during bundle adjustment using object-space control (Correct answer)
- The manufacturer pre-calibrates the sensor before delivery to the client
- Calibration performed using only onboard GPS/IMU data without ground control
Correct answer: Camera interior orientation parameters are solved simultaneously with exterior orientation during bundle adjustment using object-space control
Self-calibration (on-the-job calibration) simultaneously solves for interior orientation parameters and exterior orientation parameters within the bundle adjustment, using highly redundant imagery and ground control points.
Question 21: The f-number (f/stop) of a photogrammetric camera lens is defined as:
- The focal length divided by the effective aperture (entrance pupil) diameter (Correct answer)
- The effective aperture diameter divided by the focal length
- The ratio of image distance to object distance
- The number of optical elements in the lens system
Correct answer: The focal length divided by the effective aperture (entrance pupil) diameter
The f-number is the ratio of the focal length to the effective aperture (entrance pupil) diameter, controlling the amount of light reaching the film or sensor plane.
Certified Photogrammetrist Camera Calibration & Sensor Geometry
What is the principal point in camera calibration?
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Sources & references
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