GIS Geospatial Data Acquisition 5 — Questions and Answers
Question 1: What is the 'revisit time' (or repeat cycle) of a remote sensing satellite?
- The time required to download imagery from the satellite
- The time between successive passes over the same geographic location (Correct answer)
- The satellite's orbital period around Earth
- The time needed to process raw satellite data into a usable product
Correct answer: The time between successive passes over the same geographic location
Revisit time is the interval between two consecutive observations of the same location by the same satellite, which determines how often up-to-date imagery can be collected.
Question 2: In terrestrial laser scanning (TLS), what is a 'scan registration' or 'point cloud registration'?
- Obtaining permits to operate laser equipment in a survey area
- Aligning multiple individual scan positions into a single unified coordinate system (Correct answer)
- Registering the scanner's serial number with the manufacturer
- Converting point cloud data to a raster format
Correct answer: Aligning multiple individual scan positions into a single unified coordinate system
Registration combines multiple point cloud scans collected from different positions into one coherent dataset with a common coordinate system, typically using overlapping points or targets.
Question 3: What is 'image orthorectification' and when is it necessary?
- Enhancing image contrast for better visual interpretation
- Correcting geometric distortions caused by terrain relief, sensor tilt, and Earth's curvature to create a planimetrically accurate map (Correct answer)
- Converting a raster image to vector data format
- Removing noise and sensor artifacts from satellite imagery
Correct answer: Correcting geometric distortions caused by terrain relief, sensor tilt, and Earth's curvature to create a planimetrically accurate map
Orthorectification removes distortions from relief displacement and sensor geometry, producing an orthophoto where every pixel represents a true planimetric position suitable for accurate spatial measurements.
Question 4: What is 'GNSS augmentation' and what systems provide it in the United States?
- Adding more satellites to the GPS constellation
- Using additional reference signals to improve GNSS accuracy; WAAS (Wide Area Augmentation System) is the primary US provider (Correct answer)
- Upgrading handheld GPS devices with new firmware
- Connecting GPS receivers to cellular networks for improved timing
Correct answer: Using additional reference signals to improve GNSS accuracy; WAAS (Wide Area Augmentation System) is the primary US provider
GNSS augmentation uses ground reference stations and geostationary satellites to broadcast correction signals; WAAS provides sub-meter accuracy to compatible receivers across North America.
Question 5: What is 'hyperspectral imaging' and how does it differ from standard multispectral imaging?
- Hyperspectral captures hundreds of narrow, contiguous spectral bands; multispectral captures only a few broad bands (Correct answer)
- Hyperspectral uses higher spatial resolution; multispectral uses broader coverage
- Hyperspectral sensors operate at night; multispectral sensors require daylight
- Hyperspectral is collected from space; multispectral is collected from aircraft only
Correct answer: Hyperspectral captures hundreds of narrow, contiguous spectral bands; multispectral captures only a few broad bands
Hyperspectral sensors collect data in hundreds of narrow, contiguous spectral bands, enabling detailed material identification, while multispectral sensors typically collect only 4-12 broader spectral bands.
Question 6: What is the significance of the 'Nyquist theorem' when planning aerial image acquisition for photogrammetry?
- It determines the minimum number of ground control points needed
- It states that image overlap must be at least 50% to reconstruct continuous surface detail without gaps (Correct answer)
- It defines the maximum altitude for legal drone operations
- It specifies the minimum number of spectral bands required for vegetation mapping
Correct answer: It states that image overlap must be at least 50% to reconstruct continuous surface detail without gaps
The Nyquist theorem requires sampling at twice the frequency of the highest spatial detail, which in photogrammetry translates to needing at least 50% image overlap (typically 60-80% in practice) to reconstruct continuous surfaces.
Question 7: What does 'IMU' stand for and why is it critical in airborne LiDAR and photogrammetry systems?
- Image Mapping Unit — stores collected imagery onboard the aircraft
- Inertial Measurement Unit — records the aircraft's precise orientation (roll, pitch, yaw) needed to accurately position each data point (Correct answer)
- Integrated Mapping Utility — software that processes raw sensor data
- Infrared Measurement Unit — measures thermal emissions from the ground
Correct answer: Inertial Measurement Unit — records the aircraft's precise orientation (roll, pitch, yaw) needed to accurately position each data point
An IMU measures the aircraft's three-dimensional orientation at high frequency, which combined with GNSS position data allows each LiDAR pulse or camera exposure to be accurately georeferenced.
GIS Geospatial Data Acquisition 5
What is the 'revisit time' (or repeat cycle) of a remote sensing satellite?