DEM - Digital Elevation Models

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                                DEM—Digital Elevation Models

 Definition

 , A digital representation of a continuous variable over a two- dimensional surface by a regular
 array of z values referenced to a common datum (Where z equals elevation).

 ,Digital elevation models are typically used to represent terrain relief. Also referred to as 'digital
 terrain model' (DTM). (Where z equals elevation).

 ,An elevation database for elevation data by map sheet from the National Mapping Division of the
 U.S. Geological Survey (USGS).

 ,The format of the USGS digital elevation data sets.

 DEM Overview

 A DEM (Digital Elevation Model) is simply a digital map of elevation data. These maps, a type of
 DTM (Digital Terrain Model), are raster data meaning that they are made up of equally sized gridded
 cells each with a unique elevation.

 DEMs come in different scales and resolutions. For example, 1:24,000 scale DEM is simply a
 USGS (United States Geological Survey) 7.5’ quadrangle that has been digitized and each cell in
 the DEM represents a block of terrain 30 meters x 30 meters. The 1:250,000 scale DEM (also
 known as a 1-degree or a 3 arc-second DEM) has a resolution of 90 meters x 90 meters. DEMs
 with better resolution are available, but require large amounts of computer memory and disk space
 and are often impractical to use for large areas of land. If an individual DEM does not cover the
 entire area of interest, then multiple DEMs can be tiled together to make one large DEM.

 The projection and datum for a DEM varies. A common projection for DEMs is UTM (Universal
 Transverse Mercator) coordinates (meters) and have a specific datum associated with them.
 Elevations are usually in meters, but sometimes are in feet for areas of low relief, and are
 referenced to mean sea level.

 Uses of Elevation Data

 There are many uses and applications of DEMs. GIS (Geographic Information System) software
 such as ArcView and ARC/INFO, as well as modeling software such as WMS (Watershed Modeling
 System) can use DEMs for many engineering and scientific applications. WMS uses DEM data to
 produce watersheds which are then used to model storm events, create hydrographs, route floods
 down rivers and through reservoirs, etc.

This information can be used to design culverts, dams, detention basins and other hydraulic



  *
  From the Glossary of Mapping Terms *excerpt from The Perry-Castañeda Library Map Collection, The University of Texas at Austin
  http://www.epa.gov/ceisweb1/ceishome/atlas/learngeog/glossaryofmappingterms.html#m
DEM                                                                                                          Page 2


  structures. DEM data are commonly used to create another type of digital terrain model called a
  TIN (Triangulated Irregular Network).

 DEM Viewers

 Public domain software is available for tasks such as simply viewing a DEM -- one example is at
 http://tahoe.usgs.gov/viewers.html

 Global Mapper: formerly know as DLGV32 supports viewing of most of the popular data formats,
 including DLG-O, DRG, DOQ, DEM, DXF, SDTS, DLG, SDTS DEM, ECW, MrSID, ESRI
 Shapefiles, E00, GTOPO30, TerrainBase, ETOPO2 etc. what makes it very popular is the fact that
 it will support the latest DEM and SDTS DEM formats, including decimeter DEMs. Additional
 functionality includes Crop, reproject, and merge any combination of raster data, including DRGs.
 Free version has limited export capabilities.

Data Available in the Library
                                                           The use of the CDED is RESTRICTED to Carleton
 Canadian Digital Elevation Data: CDED                    University faculty, students, and staff for academic
 http://www.cits.rncan.gc CDED consist of an              research and teaching purposes ONLY, and is
 ordered array of ground elevations at regularly          governed by a licensing agreement with Natural
 spaced intervals. They are derived from the              Resources Canada. Users are required to sign an end
 National Topographic Data Base (NTDB)                    user license agreement BEFORE files are
 digital files at the 1:50 000 and 1:250 000              administered.

 scales, based on the divisions of the National           CDED data files must be requested at the MADGIC
 Topographic System (NTS). Carleton                       Information Desk—allow 10 days delivery time.
 University Library has access to Canadian
 Digital Elevation Data for all of Canada at scales of 1:50,000 and 1:250,000. These digital elevation
 models are based on the National Topographic Databse (NTDB) and consist of a 1 metre
 resolution grid of ground elevations recorded in metres relative to Mean Sea Level (vertical datum)
 using the NAD83 horizontal reference datum. Each CDED file is a cell that covers half of a National
 Topographic System (NTS) map sheet. As a result, each data set at the 1:50 000 and 1:250 000
 scales has an "eastern" and "western" section. The grid spacing is based on geographic
 coordinates at a minimum resolution of 3 arc-seconds and a maximum resolution of 12 arc-
 seconds, depending on the latitude. CDED are used with geographic information systems (GIS) for
 land management applications, terrain modeling, line-of-sight analysis, flood simulation, slope and
                                 aspect calculations, etc. For detailed product standards and
                                 specifications see National Resources Canada's product information
                                 booklet (PDF) available on line at: http://www.cits.rncan.gc.ca/
                                 fich_ext/1/text//products/dnec/cdedspec.pdf CDEDs are recorded
                                 in USGS ASCII format and require a number of steps to import the
                                 files into a GIS program such as ArcView. Citation Format: Centre for
                                 Topographical Information and the Canadian Forest Service, Ontario
                                    Region. Canadian Digital Elevation Data, 031g05_0100_deme
  CDED displayed in 3d Viewer [machine readable data file] Ottawa: Natural Resources Canada,
                                    [2000].
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  DMTI: Digital Elevation Model Data

  A description of the DMTI DEM files appears on the DMTI web site at
  http://www.dmtispatial.com/canadian_digital.html
  Optimal array of ground elevations at regularly
  spaced intervals. 30 and 90 metre resolutions          The use of the DMTI DEMs are RESTRICTED to
  captured from 1:50,000 NTDB. Nationwide                Carleton University faculty, students, and staff for
                                                         academic research and teaching purposes ONLY, and
  coverage based on availability of 1:50,000 NTDB
                                                         is governed by a licensing agreement with DMTI
  File format: ASCII .txt files. This format can be
                                                         Spatial. Users are required to sign an end user license
  viewed in ArcView using the Spatial Analyst or 3D      agreement BEFORE files are administered.
  Analyst extensions. Or it can be converted using
  ArcGIS Arc Toolbox into grid format. This is raster    DMTI dem data files must be requested at the
  data, with a single elevation value, in metres, for    MADGIC Information Desk—allow 10 days delivery
                                                         time.
  each cell. There are no other attributes.


  Local Coverage, Large Scale:

  National Capital Commission. DEM Files. Ottawa. 2 CD-ROMs. MDE
  These CD-ROMs are DEM files for the NCC orthophotos. Each individual orthophoto is a DEM file
  composed of x, y, and z co-ordinates spaced at 10 metre intervals. File specifications are: MTM Zone 9,
  Nad83.
  License Terms: Sign Carleton University license agreement. Data is for teaching and research purposes,
  not for commercial use.
  This is an excerpt from a file:
               321950.000 5047990.000 109.807321950.000 5048000.000 109.905321950.000
               5048010.000 110.000321950.000 5048020.000 110.256

  Yukon Coverage:

  Yukon. Dept. of Renewable Resources. Geographic Information System. RRGIS 90 Meter Digital
  Elevation Model. MDE
  The RRGIS 90 Meter DEM is a Digital Elevation Model spanning the entire Yukon and a bit beyond in
  some places. It was interpolated from the digital 1:50,000 Canadian National Topographic Database
  contour and watercourse layers using 30 meter cells (aka pixels) and then resampled to 90 meters. For
  refinement and added quality we also incorporated ancillary data not part of the NTDB. Data from the
  website:
  http://www.renres.gov.yk.ca/pubs/rrgis/data/data_desc/90m_dem.html has been copied onto a CD-
  Rom. License Terms (from the website): The DEMs are free, in both the sense of cost and of liberty. From
  the website: “You are free to use these DEMs in any way you see fit including commercial endeavours.”

   World Coverage, Small Scale:


   GeoData Software Systems. Digital terrain model (DTM): software and data sampler. Navarre, Fla.,
   GeoData Software Systems. MDE GA139.G46 1 CD-Rom. http://www.geodatas.com/dem/dtm.htm
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  Licence Terms: No restraints on use of the data. This CD-ROM contains a variety of software and
  digital data that is essential in the study of elevation information as it applies to geographic
  information system technology. The CD-ROM is divided into two sections: a software directory
  and a data directory. The software section of this CD-ROM contains numerous software titles to
  manipulate digital elevation data. When available, source code has been included with the
  executable software.

      •Elevation format converters allow the reformatting of data from/to standard DEM, SDTS
      DEM, Raster-to Vector, xyz, vector contours and numerous other formats.
      •Elevation data viewing software allows the viewing of elevation data alone or in combination
      with other types of data (i.e. imagery, Digital Line Graph [DLG]). Some of the viewing software
      also includes a full range of format conversion functionality.
      Geodetic utilities are included that allow a full range of coordinate conversions, datum
      transformations, and reprojection of geographic data.
      •Numerous demonstration, shareware and freeware viewers for elevation data are included on
      this CD-ROM.
      The digital data section of this CD-ROM contains over 3 gigabytes of compressed digital data.
      User's manuals and other documentation are included for each type of standard data format.
      The following data is included on this CD-ROM:
      •GTOPO30 elevation data provides worldwide coverage at a 30 arc second post spacing
      (approximately 1/2 nautical mile). Worldwide coverage is included on the CD-ROM.
      •ETOPO5 elevation data provides worldwide coverage at a 5 arc minutes post spacing
      (approximately 5 nautical miles). Worldwide coverage is included on the CD-ROM.
      •NAVY10 elevation data provides worldwide coverage at a 10 arc minutes post spacing
      (approximately 10 nautical miles). Worldwide coverage is included on the CD-ROM.
      •Numerous examples of Digital Elevation Model (DEM) data at 100 meter, 30 meter, and 10
      meter post spacing are included. Data is included in standard DEM, DEM SDTS, and DTED
      formats. Data sets over a variety of terrain classes are included.
      •Numerous examples of elevation data in the form of vector contour lines, are also included.
      Contour line data, in a variety of formats, is included (i.e. .DXF, DLG, XYZ).
      To allow true digital terrain modeling, many example data sets of digital line graph data,
      digital map data, and imagery are also included. In the viewing software, these can be
      draped over their corresponding elevation data to create dramatic 3-D views.


      Where can I find more information on DEMs?

      Search Subject Heading Digital Mapping in the Library Catalogue: http://
      catalogue.library.carleton.ca/

       And Keywords: DEM, digital terrain, digital elevation, etc.
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          A selected bibliography from Library Catalogue:

          Digital elevation model technologies and applications :the DEM users manual. Bethesda,
          Maryland, ASPRS, 2001. MDR GA139 .D523 2001

          Selected Internet Links:

          Digital Terrain Data: Scientific Discussion from National Geophysical Data Centre. http://
          www.ngdc.noaa.gov/seg/topo/topo2.shtml

          DIGITAL TERRAIN MODELS: DEFINITIONS, EXAMPLES, PRACTICAL USES http://
          maps.unomaha.edu/Peterson/compmapping/projects/ubbelohde/main.html

          Digital Terrain Models (DTM) may be confused with Digital Elevation Model (DEM). http://
          members.aol.com/oconrad/dgm/dgme2/dgm_e2.htm

          Glossary of GIS Terms http://www.esri.com/library/glossary/glossary.html

          Idaho Geospatial Data Center http://geolibrary.uidaho.edu/GeoLib/tutorial/
          dem_background.htm

          Lexicon of Terms for Digital Mapping Products http://cgdi.gc.ca/english/supportive/
          supportive_lexicon_e.htm

          Representing Reality in a GIS: The Raster and Vector Data Models http://
          www.geo.arizona.edu/geos256/gis/lec4.html

          Internet Guide to Downloading Data

         GIS Data Depot: http://www.gisdatadepot.com/

         Gittings, Bruce M. Catalogue of digital elevation data. Last Updated: 23-Jan-1997. http://
         www.geo.ed.ac.uk/geoinfo/dem.send

         United States Data: US Earth Resources Observation Systems (EROS):http://www.epa.gov/
         reg5ogis/gisurls2.htm

         1-Degree DEM File Finder: USGS has made available this is interactive program 1-degree
         Digital Elevation Model (DEM) data for the entire United States. Simply fill out the Latitude and
         Longitude of the site for which you need 1-degree DEM data. http://www.pes.com/pes/
         demprog.html

         USGS Geographic Data Download: http://edc.usgs.gov/doc/edchome/ndcdb/ndcdb.html

         National Geophysical Data Center is currently involved in several projects to collect public
         domain digital elevation models. http://www.ngdc.noaa.gov/seg/topo/

         Yukon Territory: http://www.renres.gov.yk.ca/pubs/rrgis/data/data_desc/90m_dem.html
DEM                                                                                                        Page 6




  World Data: Gtopo is a global digital elevation model (DEM) with a horizontal grid spacing of 30
  arc seconds (approximately 1 kilometer) http://edcdaac.usgs.gov/gtopo30/gtopo30.html

  THE PORTAL: to Global Environmental Datasets: http://www.ngdc.noaa.gov/seg/tools/gis/
  portalhome.shtml




                                                                                dem.pub HM 2003-10-28rev