GIS - Geographic Information System Geospatial Data Acquisition Questions and Answers — Questions and Answers
Question 1: A key distinction between active and passive remote sensing systems is their source of energy. Which of the following data acquisition technologies is considered an active remote sensing system?
- Landsat 9 OLI/TIRS
- LiDAR (Light Detection and Ranging) (Correct answer)
- MODIS (Moderate Resolution Imaging Spectroradiometer)
- Aerial photography with a standard digital camera
Correct answer: LiDAR (Light Detection and Ranging)
LiDAR is an active remote sensing technology because it generates its own energy in the form of laser pulses and measures the reflected returns. In contrast, passive systems like Landsat, MODIS, and standard cameras rely on detecting naturally available energy, such as reflected sunlight.
Question 2: A GIS analyst is working with a raw aerial image of a mountainous region. When overlaying vector road data, the analyst notices significant misalignment, with roads appearing displaced from their true ground positions, especially in areas of high relief. What process is required to correct these geometric distortions caused by terrain variation and sensor tilt?
- Georeferencing
- Orthorectification (Correct answer)
- Rasterization
- Spatial interpolation
Correct answer: Orthorectification
Orthorectification is the process of removing geometric distortions from aerial or satellite imagery that are caused by topographic relief and sensor tilt. This process ensures that features in the image are in their correct planimetric positions, allowing for accurate measurements of distance and area, and proper alignment with other geospatial data layers.
Question 3: A survey team needs to capture highly accurate elevation data for a small, densely vegetated watershed to support a detailed hydrological modeling project. They require a method that can penetrate the forest canopy to map the bare earth terrain beneath. Which of the following acquisition methods would be most suitable for this task?
- Structure from Motion (SfM) photogrammetry
- Collecting points with a standard consumer-grade GPS unit
- Digitizing contour lines from an existing topographic map
- Airborne LiDAR survey (Correct answer)
Correct answer: Airborne LiDAR survey
Airborne LiDAR is highly effective for this scenario because its laser pulses can penetrate vegetation canopies, recording multiple returns from a single pulse. This allows for the generation of a detailed point cloud that can be classified to distinguish ground returns from vegetation returns, resulting in a highly accurate Digital Terrain Model (DTM) of the bare earth.
Question 4: An unmanned aerial vehicle (UAV) is used to conduct a survey for a new construction project. To achieve centimeter-level accuracy without requiring a constant, real-time data link between the UAV and a base station, the surveyor records raw GNSS data on both the UAV and a stationary base. The data is corrected after the flight. What is this GNSS data collection technique called?
- Differential GPS (DGPS)
- Autonomous GNSS
- Post-Processed Kinematic (PPK) (Correct answer)
- Real-Time Kinematic (RTK)
Correct answer: Post-Processed Kinematic (PPK)
Post-Processed Kinematic (PPK) is a GNSS technique where raw satellite data is collected on the moving receiver (rover) and a stationary base station. The correction and precise positioning are calculated after the data collection is complete, making it robust against temporary losses of radio linkage that can affect RTK systems.
Question 5: Which of the following is a primary data source for creating a global Digital Elevation Model (DEM) with near-global coverage, famously collected during a Space Shuttle mission in 2000?
- The National Map (TNM)
- Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)
- Shuttle Radar Topography Mission (SRTM) (Correct answer)
- Geostationary Operational Environmental Satellite (GOES)
Correct answer: Shuttle Radar Topography Mission (SRTM)
The Shuttle Radar Topography Mission (SRTM) was a joint project that used radar interferometry aboard the Space Shuttle Endeavour to acquire elevation data for over 80% of the Earth's land surface. This mission produced a high-resolution, near-globally consistent DEM that has been a foundational dataset for countless GIS applications worldwide.
Question 6: Structure from Motion (SfM) is a photogrammetric technique used to create 3D models and point clouds from overlapping 2D images. Which of the following is a critical requirement for a successful SfM data acquisition?
- Using a camera with an integrated laser scanner.
- Ensuring all photographs are taken from the exact same location.
- Capturing a series of images with significant overlap from multiple perspectives. (Correct answer)
- Taking all photos at precisely the same time of day to ensure consistent lighting.
Correct answer: Capturing a series of images with significant overlap from multiple perspectives.
The core principle of Structure from Motion is to identify and match common features across multiple images taken from different positions and angles. A high degree of overlap (typically 60-80%) between consecutive photos is essential for the software algorithms to reconstruct the 3D geometry of the scene accurately.
GIS - Geographic Information System Geospatial Data Acquisition Questions and Answers
A key distinction between active and passive remote sensing systems is their source of energy.
Which of the following data acquisition technologies is considered an active remote sensing system?