PS Photogrammetry & Remote Sensing — 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.

Question 8: LiDAR (Light Detection and Ranging) determines distances by:

Measuring the intensity of backscattered solar radiation
Analyzing the Doppler frequency shift of emitted pulses
Recording the two-way travel time of emitted laser pulses and their returns (Correct answer)
Comparing stereo images captured from two different sensor positions

Correct answer: Recording the two-way travel time of emitted laser pulses and their returns

LiDAR emits laser pulses and precisely measures the round-trip travel time; multiplying by the speed of light and dividing by two gives the distance to the reflecting surface.

Question 9: Ground Sample Distance (GSD) in aerial or satellite imagery is best described as:

The horizontal distance between consecutive ground control points
The horizontal spacing between successive LiDAR returns on the ground
The real-world size (area) of one image pixel as measured on the ground (Correct answer)
The total ground area covered by a single flight line

Correct answer: The real-world size (area) of one image pixel as measured on the ground

GSD is the ground dimension represented by a single pixel; a smaller GSD indicates finer spatial resolution and more feature detail in the imagery.

Question 10: Which statement best describes passive remote sensing?

The sensor emits energy and measures the signal reflected back from the target
The sensor detects naturally available energy such as reflected solar radiation or thermally emitted energy from Earth's surface (Correct answer)
The system operates only in darkness using artificial illumination from ground-based sources
The platform must be within 500 m of the target surface for effective data collection

Correct answer: The sensor detects naturally available energy such as reflected solar radiation or thermally emitted energy from Earth's surface

Passive sensors record energy from natural sources—primarily reflected sunlight or long-wave thermal emission—without generating their own illumination, unlike active sensors such as LiDAR or RADAR.

Question 11: The primary purpose of aerial triangulation (aerotriangulation) is to:

Measure vertical angles between ground control points on the ground
Calculate the flying height of the aircraft using onboard altimeters
Determine spectral calibration coefficients for each camera band
Extend control from a sparse network of surveyed ground points to all photograph positions in the block (Correct answer)

Correct answer: Extend control from a sparse network of surveyed ground points to all photograph positions in the block

Aerotriangulation uses image coordinate measurements in overlapping photos and a minimal set of ground control points to solve for the exterior orientation (position and attitude) of every photo in the project block.

Question 12: Fiducial marks in a metric aerial camera are used to:

Display the aircraft heading and altitude in the corner of each photograph
Indicate the boundaries of the stereo overlap zone between adjacent photos
Define the interior photo coordinate system and locate the principal point for each exposure (Correct answer)
Record the exact GPS time of exposure on the image frame for synchronization

Correct answer: Define the interior photo coordinate system and locate the principal point for each exposure

Fiducial marks are precisely positioned reference targets fixed in the camera body that appear at the edges or corners of every exposure, establishing the image coordinate system needed to identify the principal point location.

Question 13: In LiDAR data processing, ground filtering (classification) is necessary because:

Water surfaces absorb all laser energy and contaminate surrounding ground returns
Atmospheric refraction causes ground returns to appear at systematically incorrect elevations
Returns from vegetation, buildings, and other above-ground features must be separated from bare-earth returns to generate an accurate DEM (Correct answer)
The LiDAR scanner cannot distinguish between first and last pulse returns without post-processing filters

Correct answer: Returns from vegetation, buildings, and other above-ground features must be separated from bare-earth returns to generate an accurate DEM

LiDAR pulses reflect off every surface they strike; ground filtering algorithms classify returns by height, slope, and point density to isolate the bare-earth points used for DEM generation.

Question 14: A Digital Surface Model (DSM) differs from a Digital Terrain Model (DTM) in that a DSM:

Contains only spot elevations measured at surveyed benchmarks
Represents the elevation of the highest surface including vegetation canopy and building rooftops (Correct answer)
Uses contour lines rather than a raster grid to represent elevation
Is generated exclusively from photogrammetric methods and not from LiDAR

Correct answer: Represents the elevation of the highest surface including vegetation canopy and building rooftops

A DSM captures the elevation of the topmost surface at each point—including tree canopy and rooftops—while a DTM represents the bare-earth terrain surface after classification and removal of above-ground objects.

Question 15: The exposure station (perspective center) in aerial photogrammetry is defined as:

A ground control point located at the center of the photo footprint on the ground
The spatial position of the camera's front nodal point at the instant of image capture (Correct answer)
The point on the ground directly beneath the aircraft at nadir
The focal plane location inside the camera where the image is recorded

Correct answer: The spatial position of the camera's front nodal point at the instant of image capture

The exposure station is the 3D spatial location of the camera's front nodal point at the moment of exposure; all image-forming rays are assumed to pass through this single point.

Question 16: In stereoscopic photogrammetry, x-parallax measurements are used primarily to:

Measure horizontal planimetric positions of ground features in a photo pair
Determine ground elevations from the stereo photo pair (Correct answer)
Identify camera tilt errors from systematic parallax patterns
Calculate the camera focal length from image coordinate measurements

Correct answer: Determine ground elevations from the stereo photo pair

The difference in x-coordinates of a conjugate point between the left and right photographs (x-parallax) is directly proportional to the point's elevation above the datum, enabling precise height determination from stereo pairs.

Question 17: An Inertial Measurement Unit (IMU) integrated with GPS/GNSS in an airborne photogrammetric system primarily provides:

Radiometric calibration coefficients for the camera sensor between exposures
Automatic cloud masking to flag obscured portions of each photograph
Real-time correction of ionospheric signal delay in the GPS observations
Direct measurement of aircraft roll, pitch, and yaw angles for each photo's exterior orientation (Correct answer)

Correct answer: Direct measurement of aircraft roll, pitch, and yaw angles for each photo's exterior orientation

The IMU records angular orientation (roll, pitch, yaw) at high frequency; combined with GPS-derived position, this direct georeferencing approach determines the exterior orientation of each photo without relying solely on ground control points.

Question 18: Sidelap between adjacent aerial flight strips is typically maintained at what percentage for standard mapping projects?

5–10%
20–30% (Correct answer)
50–60%
70–80%

Correct answer: 20–30%

A sidelap of 20–30% is the standard specification for aerial mapping; this ensures complete ground coverage with no gaps between strips while keeping the total number of flight lines and mission cost manageable.

Question 19: Structure from Motion (SfM) photogrammetry is distinguished from classical photogrammetry primarily by its ability to:

Produce imagery in wavelengths beyond the visible spectrum using consumer cameras
Operate effectively and produce accurate 3D results from a single high-resolution photograph
Simultaneously reconstruct 3D scene geometry and solve for camera positions from unordered overlapping image sets (Correct answer)
Achieve centimeter-level accuracy without any ground control points under all conditions

Correct answer: Simultaneously reconstruct 3D scene geometry and solve for camera positions from unordered overlapping image sets

SfM uses automatic feature detection and matching across many overlapping images to jointly solve for camera poses and sparse 3D point positions, requiring no predefined camera locations or calibration—ideal for drone-based surveys.

Question 20: Multispectral remote sensing enables vegetation health assessment primarily through:

Its inherently higher spatial resolution compared to standard RGB aerial imagery
Calculation of indices such as NDVI using near-infrared and red band reflectance values (Correct answer)
Its faster data acquisition speed compared to panchromatic or hyperspectral sensors
Its ability to image through cloud cover using microwave energy

Correct answer: Calculation of indices such as NDVI using near-infrared and red band reflectance values

The Normalized Difference Vegetation Index (NDVI) exploits the fact that healthy vegetation strongly reflects near-infrared and absorbs red light; NDVI = (NIR − Red) / (NIR + Red), with values near 1.0 indicating vigorous plant growth.

Question 21: Radiometric calibration of remote sensing imagery converts raw sensor digital numbers (DN) to:

Pixel coordinates in the camera's internal image reference frame
UTM or State Plane projected ground coordinates for map accuracy
Physically meaningful reflectance or radiance values comparable across different sensors, dates, and atmospheric conditions (Correct answer)
Classified land-cover categories derived from a supervised classification algorithm

Correct answer: Physically meaningful reflectance or radiance values comparable across different sensors, dates, and atmospheric conditions

Radiometric calibration applies sensor-specific gain and offset coefficients to raw DN values, producing top-of-atmosphere radiance or surface reflectance that is physically meaningful and enables valid multi-temporal and multi-sensor comparisons.

PS Photogrammetry & Remote Sensing

The principal point in aerial photogrammetry is best defined as:

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