Photogrammetric Data Acquisition Flashcards

A frame camera has a 5 µm pixel size and a 100 mm focal length. Flying 2,000 m above terrain, what is the approximate ground sample distance (GSD)?

Answer: 10 cm

GSD = pixel size × H / f = 0.000005 m × 2000 m / 0.1 m = 0.10 m.

To obtain a photo scale of 1:12,000 with a 152.4 mm focal length camera, what flying height above average terrain is required?

Answer: 1,829 m

H = f × scale denominator = 0.1524 m × 12,000 = 1,828.8 m.

Each photo covers 2,000 m on the ground in the flight direction and the plan calls for 60% endlap. What is the air base between exposures?

Answer: 800 m

Air base = (1 − 0.60) × 2,000 m = 800 m.

An aircraft flies at 60 m/s with a shutter speed of 1/500 s and no forward motion compensation. How far does the ground move during exposure?

Answer: 0.12 m

Ground motion = 60 m/s × 0.002 s = 0.12 m.

Why do flight planners typically raise endlap to 80% or more for urban true-orthophoto projects?

Answer: To minimize occluded areas caused by tall buildings

Higher overlap provides multiple views of each point so building lean and occlusions can be resolved.

Which sun angle condition is generally preferred for aerial photography of terrain to limit long shadows?

Answer: Sun angle at least about 30° above the horizon

Specifications commonly require a minimum solar elevation around 30° to limit shadow length.

What is the primary purpose of a gyro-stabilized camera mount during image acquisition?

Answer: To keep the camera near vertical and compensate for aircraft roll, pitch, and crab

Stabilized mounts counter aircraft attitude changes so images stay near vertical and correctly oriented.

A 150 m tall tower appears 80 mm from the principal point on a vertical photo taken 1,500 m above the tower's base. What is its relief displacement?

Answer: 8 mm

d = r × h / H = 80 mm × 150 / 1,500 = 8 mm.

In direct georeferencing, what does the lever arm describe?

Answer: The 3D offset vector between the GNSS antenna phase center and the sensor's perspective center

The lever arm is the measured spatial offset from the antenna phase center to the sensor reference point.

Boresight calibration in an integrated GNSS/IMU camera system determines what?

Answer: The small rotational misalignment between the IMU body frame and the camera frame

Boresight angles correct the angular offset between IMU and sensor axes.

What is a main advantage of post-processed kinematic (PPK) GNSS over real-time kinematic (RTK) for UAS mapping?

Answer: It does not depend on a continuous radio or cellular correction link during flight

PPK applies corrections after the flight, so link dropouts during flight do not degrade results.

Under FAA Part 107, what is the general maximum altitude for a small UAS flying a mapping mission away from structures?

Answer: 400 ft AGL

Part 107 limits operations to 400 ft AGL unless within 400 ft of a structure.

Why is airborne GNSS data quality typically degraded during steep banked turns between flight lines?

Answer: The aircraft wing can block satellites, causing cycle slips and loss of lock

Banking can mask the antenna from satellites, causing signal loss that must be resolved in processing.

A pushbroom (linear array) sensor differs from a frame camera because it:

Answer: Builds the image line by line as the platform moves, requiring continuous orientation data

Each pushbroom line has its own exterior orientation, so continuous GNSS/IMU data is essential.

A project area is 10 km wide, each image covers 2 km across track, and sidelap is 30%. What is the line spacing?

Answer: 1.4 km

Line spacing = (1 − 0.30) × 2 km = 1.4 km.

What is the main effect of a larger base-to-height (B/H) ratio in stereo acquisition?

Answer: Improved vertical (height) accuracy but potentially more occlusion

Wider intersection geometry improves height precision, though it increases viewing differences and occlusions.

Crab in aerial photography is caused by:

Answer: Failure to correct the camera orientation for aircraft heading drift due to crosswind

When crosswind makes the aircraft heading differ from its track, an uncorrected camera produces crabbed photos.

Which spectral band is most useful for distinguishing healthy vegetation in multispectral acquisition?

Answer: Near-infrared

Healthy leaf structure strongly reflects near-infrared, making NIR key for vegetation analysis.

Why does a 12-bit or 14-bit sensor offer an advantage over an 8-bit sensor in aerial imaging?

Answer: Greater radiometric resolution to capture detail in shadows and highlights

More bits per pixel record finer brightness gradations across a wider dynamic range.

When terrain elevation varies greatly within a flight line, what happens to GSD and overlap if flying height above sea level is held constant?

Answer: Over higher terrain, GSD gets smaller and overlap decreases

Higher terrain is closer to the camera, so ground coverage shrinks, reducing GSD and overlap.