PS Photogrammetry & Remote Sensing 1 — Questions and Answers
Question 1: The principal point in aerial photogrammetry is best defined as:
- The nadir point projected onto the photograph
- The foot of the perpendicular from the lens center to the image plane (Correct answer)
- The center of the exposure station above the ground
- The average ground elevation within the photograph
Correct answer: The foot of the perpendicular from the lens center to the image plane
The principal point is where the optical axis (perpendicular from the camera's front nodal point) intersects the image plane, defining the geometric center of the photograph.
Question 2: Relief displacement in an aerial photograph causes elevated objects to appear:
- Smaller than their true planimetric size
- Displaced toward the principal point
- Displaced radially outward from the principal point (Correct answer)
- Uniformly shifted toward the low end of the terrain slope
Correct answer: Displaced radially outward from the principal point
Relief displacement shifts elevated objects radially outward from the principal point—the greater the object's height and distance from photo center, the more pronounced the outward displacement.
Question 3: The photo scale at a ground point with elevation h above datum, where f is focal length and H is flying height above datum, is:
- f / H
- H / f
- f / (H - h) (Correct answer)
- (H - h) / f
Correct answer: f / (H - h)
Photo scale at any ground point equals f/(H−h); terrain at higher elevation reduces the effective flying height above ground, producing a larger-scale image for that location.
Question 4: The base-to-height (B/H) ratio in aerial stereoscopy is defined as:
- The ratio of focal length to flying height
- The ratio of air base distance between exposure stations to flying height, controlling stereo height accuracy (Correct answer)
- The ratio of forward overlap percentage to sidelap percentage
- The ratio of photo footprint width to photo footprint length
Correct answer: The ratio of air base distance between exposure stations to flying height, controlling stereo height accuracy
The B/H ratio (air base divided by flying height) governs the geometry of stereo height determination—a larger B/H improves vertical accuracy but increases relief displacement.
Question 5: In aerial photography, a standard 60% forward overlap (end lap) is maintained primarily to:
- Ensure complete stereoscopic coverage so every ground point appears in at least two consecutive photographs (Correct answer)
- Reduce the ground footprint of each individual photograph
- Eliminate sidelap requirements between adjacent flight strips
- Compensate for lens distortion at the edges of each photo
Correct answer: Ensure complete stereoscopic coverage so every ground point appears in at least two consecutive photographs
A 60% forward overlap ensures every ground point is covered by at least two adjacent photos along the strip, creating the stereo pairs required for 3D measurement and height determination.
Question 6: An orthophoto differs from a standard aerial photograph because it:
- Is captured using near-infrared wavelengths only
- Has uniform scale due to removal of relief displacement and camera tilt distortions (Correct answer)
- Shows only man-made features with natural terrain removed
- Is produced exclusively from satellite imagery at fixed orbital altitudes
Correct answer: Has uniform scale due to removal of relief displacement and camera tilt distortions
Differential rectification corrects every pixel for terrain-induced relief displacement and camera tilt, giving the orthophoto a uniform scale equivalent to a planimetric map.
Question 7: A longer focal length aerial camera, compared to a shorter focal length camera flown at the same height, produces:
- A wider angle of view and smaller photo scale
- A wider angle of view and larger photo scale
- A narrower angle of view and larger photo scale (Correct answer)
- A narrower angle of view and smaller photo scale
Correct answer: A narrower angle of view and larger photo scale
Longer focal length narrows the angular field of view (covering less ground area per photo) while increasing photo scale (scale = f/H), yielding finer detail in the imagery.
PS Photogrammetry & Remote Sensing 1
The principal point in aerial photogrammetry is best defined as: