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Svyatoslav Silin
Svyatoslav Silin

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UNIT 6: TERRAIN DATAWritten by Barry Bitters, Imaging/GIS Specialist,Lockheed Martin Information Systems ( ApplicationLearning OutcomePreparatory UnitsAwarenessLearning ObjectivesVocabularyTopicsUnit ConceptsWhat are Digital Terrain Data Sets Used for??Civil Engineering Earth Science Applications Planning & Resource Management Surveying & Photogrammetry Military Applications What are Standard Formats for Digital Terrain Data??Digital Elevation Matrix Triangular Irregular Networks Digital Contour Lines Where Can Digital Terrain Data be Found?National Oceanic and Atmospheric Administration (NOAA) U.S. Geological Survey (USGS) National Imagery and Mapping Agency (NIMA) Foreign Mapping Organizations Value-Added Vendors Engineering Firms Home-made CompetencyLearning ObjectivesVocabularyTopicsUnit ConceptsWhere can Digital Terrain Data be Found?US Navy 10' Data ETOPO5 GTOPO30 USGS DEM DCW DLG Where can Digital Terrain Data be Downloaded from on the INTERNET??Exercise: Downloading Digital Elevation Data from the INTERNET.MasteryLearning ObjectivesVocabularyTopicsUnit ConceptsPlanning a Digital Terrain Data Collection EffortWhat are the Tasks required to locate digital terrain data?What Constitutes Being the Best Data for A Job??How to Get Elevation Data into the Desired Format for a Specific Job??Exercise: Download and Reformat Elevation Data.ResourcesLiteratureSoftwareDataTutorialsSynthetic Terrain GenerationContextElevation data describes the shape of the Earth's surface and is important in thedevelopment of many human and Earth resource models. This information is used inconjunction with other data such as population, vegetation, soils, climate oragricultural information to tell a more complete story of the interaction of people andthe environment. The most fundamental type of elevation data is the raw elevation values of a singlepoint on the Earth's surface. This is a measured height of a specific point on theEarth's surface, relative to some reference plane (termed a vertical datum). Many digital elevation values stored in an orderly fashion areoften called a Digital Terrain Model (DTM). DTMs provide a convenient method forstoring and representing elevation data. DTMs are increasingly becoming the focus of attention within the larger realm ofdigital topographic data. This is because the fundamental nature of the data, and theinsight that the elevation information can provide. The insight that DTM data can addis becoming extremely valuable in numerous applications within the fields of Earth,environmental and engineering science. This is due, at least in theory, to the fact thatDTM can be used to simulate the true elevation, slope and aspect of the Earth'ssurface. Research and application of global change science, for example, is an area where theneed for quality topographic data is essential to creating Earth based intellectualmodels of real world situations. Simulation and virtual reality applications are otherareas of study that also require terrain elevation data.Example ApplicationA GIS/image processing firm has been subcontracted to create a flight simulatordatabase. Many airlines and most of the world's air forces provide simulator flighttraining capabilities for aircrew members. These flight simulator systems, althoughvery expensive have been shown to be an economical alternative to the expenses thatare encurred from actual aircraft flight time. An integral part of each aircraft flightsimulator is a geographic information system (GIS)/image database. This database isdesigned to allow the generation of realistic out-the-cockpit views of the world inreal-time. Generally, flight simulator visual databases are composed of fourseparate elements: A Geographic Information System - GIS (the Feature Database). A Ground Imagery Database (the Global Texture Database). A Library of 3-dimensional Feature Models (the Feature Model Library). A Digital Terrain Model - DTM (the Terrain Database). These database elements when digitally merged together in real-time can be used tocreate a series of terrain visualizations. The trick in flight simulator databasegeneration is to display sixty scenes per second. At the sixty scene per seconddisplay rate, the human mind perceives continuous motion instead of each individualscene. To be successful at simulator database generation, each element is createdindependently using processes that insure positional correlation between all othersimulator database elements. For example, feature information is stored in a separateprocess from those processes used to collection elevation data. However, it isessential that feature positions be compared to elevation data to insure that elevationdata does not contradict the presents of features. For example, stream data mustalways be checked to insure that elevation data does in fact portray a proper downhillflow. Elevation data for areas of open water must be checked to insure that shorelinesand water surfaces are at continuous elevations. The simulator database for which we are collecting data, is for the area around theLogan International Airport in Boston, Massachusetts. In the exercises during this unitof instruction, we will to locate elevation data for this region. Generally, three levels ofelevation detail are required: a low-resolution source, a medium-resoultion source,and a high-resolution source. These levels of resolution generally coincide with theranges of map scale: high-resolution - large scale, medium-resolution - medium scale,and low-resolution - small scale. During the exercises, we will identify variouselevation data sets for the Boston area, download some of this data from the Internet,and display and reformat some of this newly obtained elevation data. The combinedlearning experience of all the exercises will be similar to planning and data acquisitionphases of a real world project.Learning OutcomesThe following list describes the expected skills which students should master for eachlevel of training. Awareness:To attain an awareness level of this section the student must achieve a generalunderstanding of the uses of digital terrain data, knowledge of available types ofelevation data and where to obtain them. Competency:To reach a competency level of this section the student must know where toobtain various types of elevation data. The student will know how to download varioustypes of elevation data.Mastery:To attain a mastery level of the material in this section the student will havehad experience at independently planning and executing an elevation source datacollection operation. Preparatory UnitsRecommended:UNIT 38 - Digital Elevation ModelsUNIT 39 - The TIN ModelComplementary: NONE AwarenessTo attain an awareness level of this unit the student must achieve a generalunderstanding of the uses of digital terrain data, knowledge of available types ofelevation data and where to obtain them.Learning Objective:Student can define basic vocabulary relating to digital elevation data sets.Student can explain the broad uses of digital elevation data. Student can explain digital elevation data types. Student can define the sources of digital terrain data. Student will identify the global, digital, elevation data sets. Student will identify some regional digital elevation data sets of the United States. VocabularyDEM DTED DTM ETOPO5 GTOPO30 NIMA NOAA TIN USGS TopicsUnit Concepts1. What Are Digital Terrain Data Sets Used For??DTMs are used in many applications in earth science, environmental studies, and engineering. Their earliest use dates back to the 1950s when the U.S. Air Force first experimented with aircraft simulator technology. Since that time, DTMs have proved to be an important ingredient for all types of geographic modeling and the analysis of spatial topographic information. Broadly, there are five main application fields where DTMs are used: Civil Engineering Earth Science Applications Planning & Resource Management Surveying & Photogrammetry Military Applications Civil engineering: Civil engineers are mainly interested in using DTMs for cut-and-fill computations involved with road design, site planning, and volumetric calculations in building dams, reservoirs and the like. It may be pertinent to point out that owing to such overt concerns with volume and design, calling a DTM a "terrain model" has more relevance to a civil engineer than other DTM users. Earth sciences: The Earth science applications center mainly around specific functions for modeling, analysis and interpretation of the unique terrain morphology. These may include drainage basin network development and delineation, hydrological run-off modeling, geomorphological simulation and classification, and geological mapping. Generating slope and aspect maps, and slope profilesfor creating shaded relief maps is a popular usage that employs DTMs. Planning and resource management: This application of terrain data is composed of diverse fields including remote sensing, agriculture, soil science, meteorology, climatology, environmental and urban planning, and forestry, whose central focus is the management of natural resources. Examples include site location, DTM production from remote sensing, the geometric and radiometric correction of remote sensing images, soil erosion potential models, crop suitability studies, wind flow and pollution dispersion models. Surveying and Photogrammetry: One of the main objectives of employing surveying and photogrammetry is in building reliable DTMs, evaluating their accuracy towards finally producing high quality elevation data. This may be done in a number of production-related ways: field survey, or photogrammetric data capture and subsequent editing, orthophoto production, data quality assessment, or extraction from topographic maps. Military applications: The military is not only a leading consumer of DTMs, they also are a significant producer. Most military operations depend on a reliable and accurate understanding of the natural and manmade terrain. This includes a detailed modeling of elevation, slope, and aspect of the land surface. The military's use of DTMs employs a combination of the methods used by all the previous applications. Examples would include simple visualization, intervisibility analysis of the battlefield, 3-dimensional display for weapons guidance systems and flight simulation, and radar line-of-sight analyses. 2. What are the standard formats for terrain data??The basic design of a DTM is based on the structure used to represent it. DTMs can be represented by a line, or point method, or mathematically. Efficient means for mathematically describing large areas of the Earth's surface have not been found. The level of detail essential in modern earth resource monitoring systems can not be approached using the current mathematical processes available to describe irregular surfaces. A variety of data structures have been tried and tested for storing and displaying topographic surfaces, however only three have become the most popular and best explored: The rectangular grid or Digital Elevation Matrix (DEM) The triangular irregular network (TIN) The contour line structure Digital Contour Line All three methods employ representations that use a point or line model for the storage of elevation data. Two other formats will occasionally be encountered: DXF format and ASCII. These are essentially variations on free text listings of values for point geographic positions. The digital elevation matrix or rectangular grid of evenly spaced elevation values is the most commonly used digital modeling structure for a DTM. This is because the data structure of a grid shares much similarity with the file structure of digital computers. Both store elevations as a two-dimensional array (every point can be assigned to a row and a column). Because of this similarity of storage structures, the topological relations between the data points are recorded implicitly. No hard encoding of each geographic position is necessary. This streamlines both information processing and algorithm development. The triangular irregular network (TIN) model provides a network of connected triangles with irregularly spaced nodes or observation points with stored coordinates describing the point location in x, y, and z. Its major advantage over an elevation matrix is its ability to store more information in areas of complex relief, and avoid the problem of gathering a lot of redundant data from areas of simple relief. However, the disadvantage of this digital data model is that algorithm development is more difficult because of the random positioning of valid data points and a requirement for complex interpolation. The last data structure for storing digital elevation data is the contour line structure. This system is based on storing a vector version of the traditional printed contour line. This model is convenient because minimal hardware and software is required to transform hard-copy map contour lines into digital contour lines. However, algorithm development is very difficult and of the three models this one requires the most storage space. 3. Where can digital terrain data be found??Derived from medium- and small-scale mapping source materials, a significant archive of elevation data is available for the entire world. Availability of higher resolution elevation data varies based on the area of interest. The primary sources of digital terrain data are: National Oceanic and Atmospheric Administration (NOAA) United States Geological Survey (USGS) National Imagery and Mapping Agency (NIMA) formerly the Defense MappingAgency (DMA) Foreign Mapping Organizations Value-Added Vendors & Engineering Firms Home-made Elevation Data National Oceanic and Atmospheric Administration (NOAA)U.S. Navy 10-Minute Elevation DataThe U.S. Navy 10-Minute Elevation Data is a global, digital elevation data set at a resolution of 10 arc minutes. Each elevation sample is approximately 10 nautical miles apart. It was originally prepared by the Navy Fleet Numerical Oceanography Center (FNOC) at Monterey, CA. For each 10x10 arc minute area the set includes elevation, minimum elevation, maximum elevation, orientation of ridges, terrain characteristics, and urban development information. The FNOC began creating the original 10' terrain data set in the mid-1960s; work extended into the early 1970s. The main sources for the data were the US Department of Defense Operational Navigational Charts (ONC), scale 1:1,000,000. For certain regions the ONCs were not available; for such areas, selected charts from the Jet Navigation Charts and World Aeronautical Charts were used. The charts were hand-read out to paper forms, and then read by optical character reader to magnetic tape. The values were estimates from contour lines. Isometric graphs were made for quality control, such as checking terrain features. Later, other errors were corrected by the National Center for Atmospheric Research in Boulder, Colorado. Example of the uses ofthe U.S. Navy 10 Minute DataETOPO5ETOPO5 was generated from adigital data base of land and seafloor elevations on a 5-minute latitude/longitude grid. The resolution of the gridded data varies from true 5-minute for the ocean floors, theU.S.A., Europe, Japan, and Australia to 1 degree in data-deficient parts of Asia, SouthAmerica, northern Canada, and Africa. Source data used in the production of theETOPO5 data are as follows:Ocean Areas - U.S. Naval Oceanographic Office; U.S.A., W. Europe, Japan/Korea - U.S. Defense Mapping Agency Australia - Bureau of Mineral Resources, Australia; New Zealand - Department of Industrial and Scientific Research, New Zealand Remander of the world land masses - U.S. Navy Fleet Numerical OceanographicCenter (FNOC) Example of the uses ofthe ETOPO5 dataU.S. Geological Survey (USGS)Global 30 Arc-Second (GTOPO30) Elevation DataThe Global 30 Arc-Second (GTOPO30) elevation database was created by the USGS Topographic Data group at the EROS Data Center. This elevation data set provides worldwide coverage at a 30 arc second post spacing (approximately 700 to 1000 meters). This database is a composite of many data sources. The primary data source is the U.S. National Imagery and Mapping Agency's Digital Terrain Elevation Data (DTED) Level 1. The Global 30 Arc-Second elevation database consists of the generalized DTED data where ever it is available. Gaps in the generalized DTED data are filled with the best available data from other sources. One of the sources for filling gaps is elevation grids generated from the contour data provide from the Digital Chart of the World. If neither DTED or DCW data are available, then either ETOPO5 or elevation contours digitized from topographic maps were used. Image of Entire World With GTOPO30 Color Elevation TintGraphic Showing Sources of GTOPO30Graphic Showing GTOPO30 Distribution TilesDigital Chart of the World (DCW)The Digital Chart of the World(DCW) is a worldwide vector based geographic information database. This dataset, distributed on CD-ROM has a hypsography (contour) information layer thatcontains small-scale elevation information. With the proper software this data can beconverted into point elevation data. The DCW was digitized from 1:1,000,000- and1:2,000,000-scale base maps with 1,000 foot contour intervals and supplemental 250foot intervals below 1,000 feet. In areas of high relief, the contours from the DCWreadily supports a resolution better than 30 arc second elevation data. However, forareas of low relief, the 30 arc second elevation clearly shows more topographicalstructure. In other words, in relatively flat areas of the world, this information willprobably not improve on the detail of the elevation information stored in the ETOPO30data set. However, in areas of medium or high relief, the elevation data that can be generatedfrom the DCW data set can be used when resolutions better than 30 arc seconds arerequired, especially when detailed linear hydrographic information is used to producethe final digital elevation data. Some private concerns are creating substitutes elevation data from DCW for missing3 arc second elevation data. Example of DCW Elevation Data Use.Digital Elevation Model (DEM)The DigtialElevation Model (DEM) elevation data, produced for the United States and itspossessions by U.S. Geological Survey comes with sample spacing varying from 30meters for 7.5-minute DEMs to 3 arc seconds (70-90 meters) for 1:250,000 scalemaps. All DEM data are similar in logical data structure and are ordered from south tonorth in profiles that are ordered from west to east. 7.5-minute DEM dataare produced in 7.5-minute units which correspond to USGS 7.5-minute topographicquadrangle map series. 7.5-minute DEM data consist of a regular array of elevationsreferenced horizontally on the Universal Transverse Mercator (UTM) coordinatesystem of the North American Datum of 1927 (NAD 27).These data are stored asprofiles with 30-meter spacing along and between each profile. 15-minute DEM datacorrespond to USGS 15-minute topographic quadrangle map series in Alaska. Theunit sizes in Alaska vary depending on the latitudinal location of the unit. 15-minuteDEM data consist of a regular array of elevation referenced horizontally to thegeographic (latitude/longitude) coordinate syst


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