GIS Geospatial Data Acquisition 3 — Questions and Answers
Question 1: What does NDVI measure in remote sensing applications?
- Normalized Digital Vegetation Index — soil moisture content
- Normalized Difference Vegetation Index — plant health using NIR and red bands (Correct answer)
- National Digital Vegetation Inventory — census of tree species
- Non-directional Vegetation Imaging — canopy height
Correct answer: Normalized Difference Vegetation Index — plant health using NIR and red bands
NDVI uses the difference between near-infrared and red reflectance bands to quantify vegetation density and health, with values ranging from -1 to +1.
Question 2: Which orbital type do most civilian optical earth observation satellites use to ensure consistent illumination conditions?
- Geostationary orbit (GEO)
- Sun-synchronous orbit (SSO) (Correct answer)
- Highly elliptical orbit (HEO)
- Medium Earth orbit (MEO)
Correct answer: Sun-synchronous orbit (SSO)
Sun-synchronous orbit keeps the satellite's orbital plane at a constant angle to the sun, ensuring each overpass occurs at the same local solar time for consistent lighting.
Question 3: In the context of GPS accuracy, what does 'selective availability' (SA) refer to?
- The ability to select which satellites to use for positioning
- An intentional degradation of civilian GPS accuracy formerly imposed by the US military (Correct answer)
- The availability of GPS signals in remote areas
- The selective use of L1 vs L2 GPS frequencies
Correct answer: An intentional degradation of civilian GPS accuracy formerly imposed by the US military
Selective Availability was a US Department of Defense policy that intentionally degraded civilian GPS accuracy; it was permanently disabled in May 2000.
Question 4: What is 'pansharpening' in satellite image processing?
- Removing cloud cover from multispectral imagery
- Merging a high-resolution panchromatic image with lower-resolution multispectral bands to improve spatial detail (Correct answer)
- Correcting geometric distortions in satellite imagery
- Calibrating satellite sensor radiometry
Correct answer: Merging a high-resolution panchromatic image with lower-resolution multispectral bands to improve spatial detail
Pansharpening fuses a high-resolution single-band (panchromatic) image with lower-resolution multispectral bands to produce a color image with higher spatial resolution.
Question 5: A surveyor uses a total station to collect coordinates. What additional piece of equipment is needed to tie these measurements to a geodetic datum?
- A digital level
- A GPS/GNSS receiver for control points (Correct answer)
- A theodolite
- A reflective prism
Correct answer: A GPS/GNSS receiver for control points
A GPS/GNSS receiver establishes control points with coordinates in a geodetic datum, which allows total station measurements to be referenced to a global coordinate system.
Question 6: What is the key difference between 'first return' and 'last return' in LiDAR data?
- First return represents the ground; last return represents the canopy top
- First return is the initial reflection (often vegetation top); last return penetrates to the ground or lower surfaces (Correct answer)
- First return has higher intensity; last return has lower intensity
- First return is collected on the outbound pulse; last return on a separate flight
Correct answer: First return is the initial reflection (often vegetation top); last return penetrates to the ground or lower surfaces
In LiDAR, first returns typically reflect from the topmost surface (like treetops), while last returns can penetrate through gaps in vegetation to reach the ground below.
Question 7: Which technique allows UAVs to collect accurate geospatial data without requiring ground control points?
- Kinematic positioning (RTK) or Post-Processed Kinematic (PPK) GNSS on the UAV (Correct answer)
- Increasing the flight altitude above 500 feet
- Flying in a grid pattern with 90% image overlap
- Using a downward-facing barometric altimeter
Correct answer: Kinematic positioning (RTK) or Post-Processed Kinematic (PPK) GNSS on the UAV
RTK and PPK GNSS systems on UAVs enable centimeter-level positioning accuracy by using real-time or post-processed differential corrections, reducing or eliminating the need for GCPs.
GIS Geospatial Data Acquisition 3
What does NDVI measure in remote sensing applications?