Global Digital Elevation Model - Essay Example

GLOBAL DIGITAL ELEVATION MODEL by Introduction DEM as an acronym stands for digital elevation model, which pertains to the digital model (3D representation) of a given terrain’s surface mainly utilized for scientific purposes. Thus, it is mainly focused on planet terrain that includes Earth, the Moon or asteroids created with the main purpose of portraying a given terrain’s elevation data. Also referred to as DTM (digital terrain model) or DSM (digital surface model), this field of study encompasses the representation of terrain surface area including all forms of objects found within. Accordingly, a DEM representation may be in the form of a raster, height-map or grid of squares utilized when representing terrain elevation, or in the form of a vector-based TIN (triangular irregular network). DEMs are essential in geospatial data type especially in the analysis and modeling of ecological and hydrological phenomenon needed in the preservation of the environment. This paper will discuss the characteristics of the data acquisition method, including its strengths and limitations and appraise the potential of the intended DEM in a national, regional and global context. As Li, Zhu and Gold (2005) allude, a vector-based TIN (triangular irregular network) is submitted as the primary form of DEM (measured) with the former referred to as the secondary DEM (computed). In order to acquire DEM, varying techniques are utilized, for instance land surveying, IfSAR, lidar and photo-grammetry. This is in addition to the responsibility of adapting the elevation models available towards meeting the needs of existing and potential commercial users globally. As a public-private initiative, it is of great importance to many scientific and research institutions, as well as institutions of higher learning; thus, the necessary need for its commercial applicability (Li, Zhu & Gold, 2005:45). They represent terrain relief and have relevance in important applications, for instance in estimation of water and lake volume, calculation of soil erosion volumes, flood estimation and earth material estimation. It is also important to note that Digital Elevation Model is a continuous representation of elevation values over a topographic surface by a regular array of z-values, referenced to a common datum. Its successful implementation depends on the understanding of its processes. The most important aspect is to differentiate DEMs from other terrain representations that include Digital Surface Model (DSM) and Digital Terrain Model (DTM). DEM: Aspects, Influential Factors and Utility A common way in which DEMs are built is by way of data that is collected via remote sensing procedures but also may be formulated from land surveying in general. As Wilson and Gallant (2000) portray, a key avenue for the utility of DEMs is in existing geographic information systems (GISs) that is the most common foundation on which digitally produced relief maps are formulated. Accordingly, DEMs are not only vital in the field of land surveying, instrumental in the fields of city modeling, visualization applications and landscape modeling but also are also useful in land-use studies, drainage or flood modeling and geological applications amongst others (Wilson and Gallant, 2000:7). From the mentioned factors, it is clear that the profession of mapping mostly utilizes such techniques, preparing models in a variety of ways. The most important factor is remote sensing as opposed to the more strenuous direct survey data. To be noted is that a DEM’s quality regards the standard of accurate elevation as presented by each pixel taken (absolute accuracy) and how such accuracy is represented in terms of relative accuracy (morphology). A number of factors play vital roles with regard to quality representation of DEM-derived production, for instance the terrain’s surface roughness, the pixel size (grid resolution), vertical resolution, sampling density (collection method of elevation data), interpolation algorithm, reference 3D products and terrain analysis algorithm (Wilson & Gallant, 2000:10-11). In terms of obtaining the relevant elevation data utilized in the creation of DEMs, Jensen (2006) points to several methods utilized including block adjustment (from imagery acquired via optical satellites), lidar, range imaging, inertial surveys, stereo photo-grammetry (by way of aerial surveys) and topographic maps. Others include the Doppler radar, surveying and mapping drones, topographic maps, inter-ferometry (acquired from radar data) and Kinematic (real time) GPS. Common utility of DEMs is in the creation of relief maps, 3D flight planning, GPS, geographic information systems, the rendering of 3D visualizations, the extraction of terrain parameters (geo-morphology). Other things created by the DEMs include rectification of satellite imagery (aerial photography), terrain analysis, base mapping, line-of-sight analysis, archaeology, advanced driver assistance systems (auto safety), flight simulation, surface analysis, precision forestry and mapping, as well as in intelligent transportation systems (Jensen, 2006:31). The DLR/Astrium-tandem-X Space Mission While different mapping agencies mostly national entities produce their own DEMs, often of better quality and resolution, a majority of these productions are usually purchased. As Bartusch (2014) avers, the pricing is frequently prohibitive with large corporate entities and public authorities acting as the only entities able to purchase at such price rates. Accordingly, a majority of DEMs are usually a production of existing lidar dataset programs of a national status. However, they are also available in free versions, mainly gained from: NASA’s DTM (digital terrain model), the Mars Global Surveyor’s MOLA (Mars Orbiter Laser Altimeter) instrument. They are mainly focused on the planet Mars and represented as the Mission Experiment Gridded Data Record. The DLR/Astrium-tandem-X Space Mission pertains to the utility of two (almost identical) earth observation satellites – the TanDEM-X and TerraSAR-X – each equipped with SAR technology. The Tandem-X (TerraSAR-X add on for DEM) mission, a public-private initiative between Astrium, and the German government utilizes synthetic Aperture Radar (SAR), in the two satellites’ observation of earth’s surface. Astrium, through its subsidiary Infoterra GmbH, holds exclusively WorldDEM’s commercial marketing rights. This is not only possible during the daytime but also when visibility is obscured by the presence of cloud cover and/or darkness (nighttime). Tandem-X is equipped with two antennas, which work like human eyes in the production of 3D visualization making it the novel system equipped with the capacity to generate 3D elevation models of earth’s entire surface terrain (Bartusch, 2014). Moreover, both satellites are equipped with varying devices influential in the synchronization of the two radar instruments and an on-board navigation controller that works autonomously. As Krieger et al. (2011) informs, in completing the system, the presence of a ground segment, which is highly complex, is vital in supporting the ongoing operational procedures of the entire mission. The two satellites are programmed to complement each other flying in formation in their orbit positions at 514 kilometers above earth. Uniqueness is their close proximity flying together closely at estimated closest levels of less than 200 meters apart; hence, enhancing their overall operational capabilities (Krieger et al., 2011:3-4). Accordingly, the two have enabled various fields of study in terms of generation of global digital elevation models with vertical resolutions of 2 meters, and horizontal grids measuring 12 by 12 meters. Scientific projection of ultimately gaining a global 3D elevation model was projected to be three years after successful launch of both where the TanDEM-X being the last to launch in 2010. Thus, the two entail the novel configurable SAR interferometer currently in orbit (Krieger et al., 2011:5). During bi-static operations, as Hajnsek and Moreira (2006) portray, one of the two emits radar signals, with the earth’s surface producing backscatter that is received by both; hence the complementary nature of the two equipments. In order to cover the entire global surface area, the mission necessitated a total of 3 years of parallel operations where both satellites fly in their configured (charted course) formation (Hajnsek and Moreira, 2006:22). In such a period, the two satellites working in tandem can be able to survey the earth entire land surface. Data generated within such a process, is more than anything that has been recorded generated at 1.5 petabytes. Accordingly, the project displays a step in the right direction, given that the procedures logically follow other prior international radar missions i.e. SRTM (shuttle radar topography mission) and X-SAR (X-band synthetic aperture radar). Additionally, in terms of logic, it also follows the successfully completed national TerraSAR-X project (Hajnsek & Moreira, 2006:25). The Generation and Utility of DEM The project, as Eineder, Krieger and Roth (2006) present is a public-private partnership mission, which continues to play a crucial role; abate the limited time of operation within earth’s orbit. Accordingly, the partnership engages the private sector, which provides crucial financial backing for the expensive missions. Thus, while EADS Astrium continues being in charge of the development, building and launch of the satellites, DLR is ultimately responsible for overall management of the mission (Eineder, Krieger & Roth, 2006:13). An integral part of the satellite payload is the Tracking, Occultation and Ranging Experiment (TOR) consisting of a dual-frequency GPS receiver. The receiver subsequently permits the overall determination of the satellites’ orbit range to within a few centimeters in addition to the utility in the measurement of radio occultation within the earth’s lonosphere and atmosphere (Eineder, Krieger & Roth, 2006:15). As projected in its primary mission, the DLR/Astrium-tandem-X Space Mission continues in its quest of generating WorldDEM, which pertains to the consistent global DEM that has greater capacity than existing DTED Level 2 specifications. Accordingly, there will be a correspondence between WorldDEM and existing DTED Level 3 regarding better height accuracy than 2m (relative) and post-spacing that is better than 12 meters (Eineder, Krieger & Roth, 2006:29). DEMs that post greater accuracy than the aforementioned DTED Level 2 category are accordingly referred to as High Resolution Terrain Information (HRTI) DEM. Thus, with its unprecedented features i.e. quality, coverage and accuracy, the WorldDEM provides a consistent DEM pertaining to Earth’s complete land surface. This features a vertical accuracy of 4m (absolute) and 2m (relative) encompassed within an approximately 12*12 square meter horizontal raster faintly varying according to the geographic latitude represented (Eineder, Krieger & Roth, 2006:33). The first publishing of 3D imagery of earth’s surface terrain generated was by researchers at Oberpfaffenhofen (Germany) Aerospace Center facility where a group of Russian islands selected for the mission’s novel test. As Krieger et al. (2011) portrays, based on subsequent data generated, DLR continuously conducts several processing steps, aimed at coming up with a seamless and global DEM of the Earth’s landmass. This generation of data is projected to continuously meet various specifications, as envisaged by the different commercial utilities to be undertaken each serving a specific field of focus. Commercialization of the DEM product is viewed as Astrium’s (the private partner) ultimate goal, in aid of the aforementioned variety of uses (Krieger et al., 2011:8). It is critical to note that drawbacks are present because of the fact that space-borne SAR data usually is acquired within ‘repeat-pass mode’; meaning that imagery is taken from a singular orbit position but on separate dates. However, such acquisition of data in a sequential manner has its drawbacks of which two are of major concern (Krieger et al., 2011:10). The first pertains to the fact that in-between the periods of imagery taking the targeted scenes can change. Given this fact, this temporal de-correlation can even result in loss of entire coherence. To be noted is that besides vividly clear changes in terms of land coverage, even varying subtle processes, i.e. plant growth throughout phenological active periods can result in considerable deterioration of DEM quality. Maune (2001) observes that this can even lead to areas of pure noise when extreme cases are reached. In addition to this drawback, the second one pertains to the fact that water vapor present within the troposphere influences overall velocity of light. Accordingly, such an effect is usually characterized by the presence of a spatial correlation length ordered in kilometers, which fully decorrelates during repeat cycle time spans of common remote sensing satellites placed on earth’s low orbit space (Maune, 2001:536). This subsequently results in the signal’s atmospheric path delay differing, for image acquisitions taken at dates, which are separated by some weeks. To duly avoid the two aforementioned major drawbacks, there is need for the two SAR images to be acquired simultaneously. This means that no temporal changes can take place despite the prevailing atmospheric conditions with the related phase term canceling out because of taking the difference in imagery acquired (Maune, 2001:540). Such data (single-pass) are also usually gathered by way of airplane utility those of which are outfitted with two antenna systems. These antenna systems are mounted in such a way that there is establishment of an across-track baseline. In choosing the orbit dimensions, it is noted that the orbits of such satellites, ought to never cross to enable safe navigation of spacecraft. Accordingly, the choice of orbit dimension, range and height amongst other factors, are chosen in a manner in which the two satellites trail along a helix configuration. The result of this choice, dependent on the latitude is that the spatial baseline changes in a continuous manner; hence the need to measure these changes with millimeter accuracy (Maune, 2001:543). This is important in meeting the required specifications of the DEM product under production. However, Hajnsek and Moreira (2006) assert that the prevention of DEM generation is influenced by the vanishing aspect of effective baselines within across-track directions for specific latitudes. Thus, in order to account for the occurrence, the choice of helix setup was in consideration of either descending or ascending orbits ensuring an adequately large effective baseline (Hajnsek and Moreira, 2006:65). This configuration ultimately leads to a considerable along-track baseline, whose magnitude is between 0 meters (in Polar Regions) and 1000 meters (at the equator). This specific kind of setup is vital in the overall determination of various objects’ velocity. However, with respect to DEM generation processes, this setup is disadvantageous given the fact that there is a time delay/ lapse of up to 50 milliseconds (Hajnsek and Moreira 2006:70). Because in very high altitudes water is frozen, there is no decorrelation occurrence after a few milliseconds time frame, as is the case on water surfaces on earth. Accordingly, because of the aforementioned fact, no useful signal can be received from liquid water surfaces such as lakes, rivers or oceans; with only noise emanating at such related locations (Hajnsek & Moreira, 2006:73). Thus, in order to mask out such surface areas when under procedural measures, there is considerable effort that is spend from DLR facility with the aim of ultimately detecting such effects by way of an automated manner. As earlier aforementioned, due to the private-public partnership of this endeavor, Astrium as the private entity is poised on eventually commercializing the DEM product. Such a complex program is only achievable after the data acquisition and (some) processing is carried out. However, during the mission, there is provision of intermediate data to the radar community mainly for scientific purposes. The core concept behind the mission’s basic operations concept pertains to the handling of the two satellites as independently as it may be possibly achievable (Bartusch 2014). The TanDEM-X pertains to a highly inventive radar mission based on technology that is proven further augmented by a number of fundamental improvements. In tandem with concepts of optimized data acquisition, as Maune (2001) avers, a global DEM whose quality thus far know is only from local available DEMs, is under production (generated) (Maune, 2001:547). The greater process is based on both content- and global-driven processing of DEMs, and not the singular scenes. Of importance in this scenario is the fact that the task scheduled for a complete global DEM generation is only achievable after several years when there is successful mapping of the whole earth. In this regard, the success criteria pertain to gapless (wholesome) coverage of the earth with DEM products augmented by the provision of gapless coverage within each singular DEM products (in order to avoid both layovers and shadow projection) as well as victorious DEM reconstruction in terms of correct resolution projection of the integer phase ambiguities (Maune, 2001:547-48). In conclusion, DEM as an acronym stands for digital elevation model, which pertains to the digital model (3D representation) of a given terrain’s surface mainly utilized for scientific purposes. From the perspective of meteorological field, land surveying and mapping to the aspect of Global positioning and security and defense, the mission is indeed a great step into the future in terms of effectiveness, efficiency and overall comprehensiveness. A common way in which DEMs are built is by way of data that is collected via remote sensing procedures but also may be formulated from land surveying in general. A key avenue for the utility of DEMs is in existing geographic information systems (GISs) that are the most common foundation on which digitally produced relief maps are formulated. The most important factor is remote sensing as opposed to the more strenuous direct survey data. DEM’s quality regards the standard of accurate elevation as presented by each pixel taken (absolute accuracy) and how such accuracy is represented in terms of relative accuracy (morphology). It is important to note that DEM is essential in geospatial data type especially in the analysis and modeling of ecological and hydrological phenomenon needed in the preservation of the environment. Reference List Bartusch, M. 2014. TanDEM-X. DLR Space Administration [German Aerospace Center: Space Administration, Earth Observation], retrieved from: http://www.dlr.de/rd/en/desktopdefault.aspx/tabid-2440/3586_read-16692/ Eineder, M., Krieger, G. & Roth, A. 2006. First Data Acquisition and Processing Concepts for the TanDEM-X Mission. Oberpfaffenhofen, Germany German Aerospace Center DLR. Hajnsek, I. & Moreira, A., 2006. TanDEM-X: Mission and Science Exploitation during the Phase A Study. Proceedings of EUSAR 2006, Dresden. Hohle, J. & Marketa, P. 2011. Assessment of the Quality of Digital Terrain Models. Jensen, J. R. 2006. Remote Sensing of the Environment: An Earth Resource Perspective, (2nd Ed). Prentice Hall. Krieger, G., Zink, M., Schulze, D., Hajnsek, I., Moreira, A. 2011. TanDEM-X: Mission Overview and Status. Proceedings of the International Conference on Spacecraft Formation Flying Missions & Technologies (SFFMT). St-Hubert, Canada, (pp.1-10). Li, Z., Zhu, Q. & Gold, C. 2005. Digital terrain modeling: principles and methodology. Boca Raton: CRC Press. Maune, D.F. 2001. Digital elevation models Technologies and Applications: The DEM Users Manual. ASPRS. Soergel, U., Karsten, J. & Lukas, S. 2010. The TanDEM-X Mission: Data Collection and Deliverables. Wilson, J.P. & Gallant, J.C. 2000. Terrain Analysis: Principles and Applications (Eds.). New York: Wiley. Read More