Lecture 04 CE797 F24.pdf

Faculty of Engineering Lecture (4) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani...

Faculty of Engineering Lecture (4) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (2 of 3) GIS Applications in Civil Eng 2.4.3 Remote Sensing Methods These methods use active and passive sensors mounted on airborne platforms such as planes and drones, or spaceborne platforms (satellites or space shuttles). Optical and microwave portions of the electromagnetic (EM) spectrum are used by remote sensing methods. Optical EM spectrum includes visible light, near infrared and shortwave infrared radiations. Imageries are usually taken using optical EM radiations. The acquired overlapping imageries are used to generate DEM by stereo photogrammetry method as we will see below. Radar imageries use microwave EM radiations. The acquired overlapping imageries are used to generate DEM by stereophotogrammetry method as we will see below. Non imaging remote sensing methods include Radar Altimetry and Lidar. Radar (Radio Detecting and Ranging) altimetry uses active sensors to measure elevations. Lidar (light detection and ranging) uses active sensors with optical electromagnetic energy. (1) Stereophotogrammetry Method Stereophotogrammetry is the classical way to produce DEMs from stereo pairs obtained by optical remote sensing, which is a group of passive remote sensing approaches. DEMs can be derived from stereo pairs of aerial and satellite images. Stereophotogrammetry can be done manually or automatically. The primary advantage of stereo imagery is the ability to extract geographic features in 3D such as buildings, roads, manmade structures and other terrain features. ▪ Manual method: an operator looks at a pair of stereophotos through a stereoplotter and must move two dots together until they appear to be one lying just at the surface of the ground ▪ Automated method: an instrument calculates the parallax A stereopair of optical image displacement of a large number of points to make relative and absolute orientation of the stereopair. Automatic collection of DEM points is performed. ▪ The basic idea of stereophotogrammetry is as follows. Suppose that there are two images of the same object taken from two different positions. By reconstructing the geometric parameters of the photographing condition (camera parameters, position, and orientation), then a stereo model of the object can be constructed, and the A stereopair of radar image coordinates (x, y, z) of any point can be computed. Stereo correlations, or image matching, are used for this purpose. ▪ Automated photogrammetric generation of DEMs can be done either by processing digitized stereo pairs from an analog camera, or by handling digital stereo images. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 1 of 5 Faculty of Engineering Lecture (4) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (2 of 3) GIS Applications in Civil Eng ▪ To generate DEMs, satellite stereo images from various platforms can be utilized, such as SPOT, Ikonos, QuickBird, ALOS and the like. ▪ Various optical satellite sensors are used for DEM generation, such as Quickbird, IKONOS (2-5 m resolution), the Pleiades 1A / 1B constellation (1m. resolution), WoldView-2 and GeoEye- 2 (1-2 m resolution), the Japanese Advanced Land Observing Satellite (ALOS) PRISM (2.5 m), Indian Cartosat (2.5 m), the French SPOT satellite (5-10 m), and ASTER (15-30 m). ▪ A very useful source for world-wide medium resolution (30 m) free DEM data is the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), launched in 1999, which carries 15 channels, with 4 bands at 15 m resolution, 6 at 60m, and 5 at 90m. The VNIR sensor has in total four bands, of which one is back-ward looking, allowing the generation of DEMs with a pixel resolution of 15 m and a vertical accuracy less than 20 meters. The DEMs generated from ASTER images are now freely available from Earthdata webservice at https://search.earthdata.nasa.gov/search. ▪ The application of DEMs from very high resolution images (Quickbird or IKONOS) in detailed studies is hampered by the high acquisition costs (30-50 USD/km2). The recently launched high resolution data from PRISM (ALOS) and CARTOSAT-1, both with 2.5 m resolution, both with two panchromatic cameras that allow for near simultaneous imaging of the same area from two different angles (along track stereo) are able to produce highly accurate DEMs, at costs lower than 10 USD/km2. ▪ It is common practice to use stereo pairs from the same platform. In particular, the quasi- global ASTER GDEM was produced by the photogrammetric processing of stereo pairs from the Terra satellite. At the same time, is it possible to create a DEM using two photos obtained from different satellites; for example, taking one image from Landsat MSS and another one from NOAA AVHRR. ▪ Recently, unmanned aerial vehicles (UAVs), or drones, have been increasingly used for remote sensing. UAVs can be equipped with various types of sensors, such as visible band, multispectral, and thermal cameras; laser scanners; SAR; and so on. ▪ UAVs and UAV photogrammetry introduced a low-cost alternative to manned aerial photogrammetry. In particular, UAV images are utilized for derivation of high- and very high- resolution DEMs. (2) Laser altimetry (Lidar) Method Laser altimetry or LiDAR (Light Detection And Ranging) is a group of active remote sensing techniques. Its general scheme is as follows: A laser altimeter emits laser pulses, which are reflected from the ground surface; part of the reflected radiation returns and is detected by the instrument; then, the distance from the ground surface can be calculated considering the speed of light. Satellite laser altimetry is widely used for DEM generation. ▪ Laser altimetry exists in two main forms: light detection and ranging (LiDAR) aerial survey, and satellite laser altimetry. ▪ LiDAR aerial surveys are commonly applied to create high-resolution DEMs of both the land surface and the seafloor, down to 70 m in clear waters. ▪ LiDAR aerial surveys offer fast DTM-based solutions for large-scale mapping. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 2 of 5 Faculty of Engineering Lecture (4) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (2 of 3) GIS Applications in Civil Eng ▪ More detailed DEMs are nowadays derived using LiDAR. Normally LiDAR point measurements will render so-called Digital Surface Models (DSM), which contains information on all objects of the Earth’s surface, including buildings, trees etc. Through sophisticated algorithms, and final manual editing, the landscape elements are removed and a Digital Terrain Model (DTM) is generated. ▪ LiDAR has become the standard method for the generation of high resolution DEMs in many developed countries. LiDAR can measure distances very precisely; therefore, a DSM with a high density of points can be obtained with accurate elevation values. The attainable elevation (vertical coordinate) accuracy with LiDAR can be in the order of 3 cm for well- defined target surfaces. ▪ The average costs of LiDAR ranges from 300 to 800 US$/km2 depending on the required point density. (3) Synthetic Aperture Radar (SAR) Method SAR is a group of active remote sensing techniques. The SAR antenna transmits electromagnetic radiation at microwave frequencies, which is reflected from the area of interest. Then, the radar echoes are received by the instrument’s receiver. SARs are usually mounted on moving platforms, such as aircraft and spacecraft. ▪ SAR-based DEMs are utilized in medium- and small-scale mapping of large areas. ▪ To produce DEMs, four SAR techniques can be applied: (1) interferometry (2) stereoscopy using stereo pairs of radar images, (3) polarimetry and (4) clinometry. ▪ The interferometric SAR (InSAR) techniques are in most common use. InSAR utilizes phase- difference measurements derived from two radar images, which can be obtained by one-pass or two-pass collection modes. In the one-pass mode, a platform has two SARs but only one of them, the primary antenna, transmits pulses; the radar echoes are received by both antennas: primary and secondary. In the two-pass mode, a platform with a single SAR makes two flights over the target area. ▪ The InSAR technique was applied in the Shuttle Radar Topography Mission (SRTM) to prepare SRTM30, SRTM15, SRTM3 and SRTM1 Global DEMs with 30”, 15”, 3” and 1” resolution respectively. These resolutions correspond to about 1 km, 500 m, 100 m and 30 m respectively. These are medium-resolution DEMs of the earth, and are suitable for medium to small scale mapping of large areas. ▪ The large one-pass spaceborne SAR interferometer, based on two similar radar satellites, TerraSAR-X and TanDEM-X, was intended for production of the WorldDEM, the recent quasi- global high-resolution DEM. ▪ There are other satellite-based SAR systems Used to produce DEM such as the European ERS- 1 and the Canadian Radarsat satellites. ▪ DEMs are also derived using radar satellites such as RADARSAT, TerraSAR-X, ALOS PALSAR, ERS-1 and 2, ENVISAT). Synthetic Aperture Radar Interferometry (InSAR) can be used for the generation of Digital Elevation Models, but in practice it is mostly used for detecting changes in topographic heights, related to different hazardous geological processes, such as land subsidence, slow moving landslides, tectonic motions, ice movement and volcanic activity. Textbook: Introduction to Geographic Information Systems, Kang-tsung Chang, McGraw-Hill (2019) Page 3 of 5 Faculty of Engineering Lecture (4) CE 797: Special Topic Civil Eng. Department Digital Elevation Models (DEMs) Digital Elevation Models and Dr. Suhail A. Almadani Chapter (2) (2 of 3) GIS Applications in Civil Eng (4) Satellite Radar Altimetry Method Satellite radar altimetry is an active remote sensing approach used to calculate heights on the earth by sending out short pulses of microwave energy in the direction of interest with the strength and origins of the received back echoes or reflections measured. The signal is corrected for a number of potential error sources, for example, the speed of travel through the atmosphere and small changes in the orbit of the satellite. ▪ Applications include calculating the height of the land, ocean, and inland water bodies. An example is the Jason-2 Ocean Surface Topography Mission, launched in 2008, that carries the Poseidon-3 Radar altimeter, Advanced Microwave Radiometer (which allows the altimeter to be corrected for water vapor in the atmosphere), and instruments to allow the satellite’s position in space to be determined accurately. ▪ Satellite radar altimeters are used to measure the shape of the sea surface, and to reveal marine gravity anomalies. Ship track echo-sounding data are too sparse to fully define the submarine topography. Current bathymetric charts are inadequate for many of these applications because only a small fraction of the seafloor has been surveyed. (5) Airborne Optical Sensing of Bathymetry Method Airborne optical sensing of bathymetry is a group of passive remote sensing approaches. They are based on a principle that the total amount of radiative energy reflected from a water column is a function of water depth. The water depth is estimated from brightness values of the image. ▪ This can be used to create DEMs of shallow seafloors, which may be hazardous for navigation and echo sounding. The depth is estimated from brightness values of the image. ▪ These approaches may be applied for clear waters only. They can be used to create DEMs of shallow seafloors, which are hazardous for navigation, and thus echo sounding. Such DEMs may be used in large- and medium-scale mapping. (6) Echo Sounding Method Echo sounding is a group of sonar hydrographic, active remote sensing techniques. They are used to determine the distance between the water surface and underwater topography by transmitting sound pulses into water. The depth of water is determined from the time lag between the emission and return of a pulse, as well as the sound speed in the water. ▪ Echo sounding can be performed by single-beam, multibeam, and side scan systems. Common single-beam echo sounders transmit a single beam oriented toward the vessel’s nadir. They usually utilize short (