PS Photogrammetry & Remote Sensing 2 — Questions and Answers
Question 1: 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 2: 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 3: 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 4: 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 5: 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 6: 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 7: 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.
PS Photogrammetry & Remote Sensing 2
LiDAR (Light Detection and Ranging) determines distances by: