Photogrammetry & Remote Sensing Flashcards

The principal point in aerial photogrammetry is best defined as:

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.

Relief displacement in an aerial photograph causes elevated objects to appear:

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.

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:

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.

The base-to-height (B/H) ratio in aerial stereoscopy is defined as:

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.

In aerial photography, a standard 60% forward overlap (end lap) is maintained primarily to:

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.

An orthophoto differs from a standard aerial photograph because it:

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.

A longer focal length aerial camera, compared to a shorter focal length camera flown at the same height, produces:

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.

LiDAR (Light Detection and Ranging) determines distances by:

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.

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

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.

Which statement best describes passive remote sensing?

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.

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

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.

Fiducial marks in a metric aerial camera are used to:

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

Multispectral remote sensing enables vegetation health assessment primarily through:

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.