Questions and Answers
These DEMs are based on the ______ ellipsoid.
WGS84
LiDAR can detect multiple ______ signals for a single transmitted pulse.
return
Digital orthophotos have the displacement caused by camera tilt and terrain ______ removed.
relief
The USGS began producing digital orthophoto quads DOQs in ______.
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The standard USGS DOQ format is a ______-minute quarter quadrangle.
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DOQs are useful for checking the accuracy of map layers such as ______ and parcel boundaries.
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Land cover data are typically classified and compiled from ______ imagery.
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The NLCD databases use a ______-class scheme classified from Landsat images.
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A bi-level scanned file contains values of ______ or 0.
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Bi-level scanned files are usually made for the purpose of ______.
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A digital elevation model (DEM) consists of an array of uniformly spaced ______ data.
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Traditional methods for producing DEMs include using a ______ and stereo pairs of aerial photographs.
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DEMs can also be interpolated from the contour lines of a ______ map.
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Optical sensors like Terra ASTER and SPOT 5 are used to generate ______ satellite images.
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ASTER provides a nadir view and a backward view within a ______.
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SPOT 5 DEM has a spatial resolution of ______ m.
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InSAR uses two or more SAR images to generate ______ of the reflective surface.
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SRTM DEMs are derived from SAR data collected by two radar antennas placed on the ______.
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Light detection and ranging (LiDAR) is one of several new techniques for DEM ______.
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New techniques include photogrammetry and ______ laser scanning.
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A digital raster graphic (DRG) is a scanned image of a USGS ______ map.
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Maps to be digitized are typically scanned at 300 or ______ dots per inch (dpi).
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Rasterization converts ______ data to raster data.
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USGS DRGs are georeferenced to the UTM ______ system.
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Many popular graphic files are in ______ format, such as TIFF and JPEG.
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Vectorization converts raster data to ______ data.
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The first step of rasterization sets up a raster with a specified ______ size.
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The value of cells that correspond to points in rasterization is set to ______.
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In vectorization, line thinning reduces the width of raster lines to ideally ______ pixel.
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Line extraction is the process of determining where individual lines ______ and end.
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SRTM DEMs cover over 80 percent of the landmass of the Earth between 60° N and ______ S.
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In the United States, elevation data from SRTM DEMs is spaced ______ arc-second apart between 0° and 50° latitude.
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For other countries, SRTM DEMs are available at a ______-meter resolution.
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Higher resolution DEMs can be created from SAR images collected by ______, TerraSAR-X, and RADARSAT-2.
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TerraSAR-X operates at an orbit height of ______ km.
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LiDAR data for DEM generation has increased significantly since the ______-1990s.
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The basic components of a LiDAR system include a laser scanner, GPS, and an ______ Measurement Unit (IMU).
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Using the time lapse of the laser pulse, the distance between the scanner and the ______ can be calculated.
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A major application of LiDAR technology is the creation of high resolution DEMs with a spatial resolution of ______ to 2 meters.
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The pulse generator in a LiDAR system emits rapid laser pulses with a wavelength between ______ and 1.6 µm.
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Study Notes
Digital Elevation Models (DEMs)
- A DEM is an array of uniformly spaced elevation data crucial for terrain mapping and analysis.
- Traditional DEM production involves the use of stereoplotters and stereo pairs of aerial photographs; this method is accurate but labor-intensive and requires skilled operators.
- Contour lines from topographic maps can also be used to interpolate DEMs.
DEM Generation Techniques
-
Optical Sensors:
- Utilizes two or more optical satellite images from different angles.
- Key Optical Sensors:
- Terra ASTER: 30 m spatial resolution.
- SPOT 5: 20 m spatial resolution.
- High-resolution satellite images like World-View can also generate DEMs if stereo pairs are available.
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InSAR:
- Generates elevations using synthetic aperture radar (SAR) images.
- SRTM DEMs from 2000 cover over 80% of Earth's landmass and provide elevation data at different resolutions (1 arc-second ≈ 30 m at midlatitudes).
- Newer SAR data from Sentinel-1, TerraSAR-X, and RADARSAT-2 offer higher resolution DEMs (down to 1 m).
-
LiDAR:
- Laser scanner technology mounts on aircraft to produce detailed DEMs.
- Capable of generating high-resolution DEMs (0.5 to 2 m spatial resolution) with georeferencing based on WGS84.
- Can capture multiple height levels (e.g., ground vs. canopy elevation) using return signals.
Other Raster Data Types
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Digital Orthophotos:
- Digitized images of aerial photographs where terrain relief displacement is removed.
- Created by the USGS since 1991 with a ground resolution of 1 m.
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Land Cover Data:
- Classified from satellite imagery, released by USGS as NLCD databases (2001, 2006, and 2011) using a 16-class scheme with 30 m spatial resolution.
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Bi-Level Scanned Files:
- Scanned images with binary values (1 or 0), often used for digitizing boundaries from maps.
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Digital Raster Graphics (DRGs):
- Scans of USGS topographic maps with a ground resolution of 2.4 m, georeferenced to the UTM coordinate system.
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Graphic Files:
- Various maps and images can be stored in raster formats like TIFF, GIF, and JPEG.
Data Conversion and Integration
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Raster and vector data models remain separate in practice although integrated models have been proposed.
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Rasterization:
- Converts vector data to raster data in three steps: setup raster grid, assign values to relevant cells, and fill polygon interiors.
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Vectorization:
- Converts raster data to vector data, involving line thinning, line extraction, and topological reconstruction.
- Essential for maintaining geometrical integrity when transforming data types.
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Description
Explore the fundamental concepts of Digital Elevation Models (DEMs) and their generation techniques. This quiz covers methods like optical sensors and InSAR, detailing equipment used and their resolutions. Brush up on your knowledge of terrain mapping and analysis through various DEM production techniques.
