7.5-Minute Digital Elevation Model (DEM)

Digital Elevation Model (DEM) is the terminology adopted by the USGS to describe terrain elevation data sets in a digital raster form. The 7.5-minute DEM (30- by 30-m data spacing, cast on a Universal Transverse Mercator (UTM) projection) provides coverage in 7.5- by 7.5-minute blocks. Each product provides the same coverage as a standard USGS 7.5-minute quadrangle without over edge.  This file contains metadata reports for both the standard USGS format DEMs and the Arc/INFO lattice files available from the Georgia GIS Data Clearinghouse.

Information about individual 7.5-Minute DEM records can be obtained from the EROS Data Center. 

Table of Contents

Identification Information

  Identification_Information:

    Citation:

      Citation_Information:

        Originator: U.S. Geological Survey

        Publication_Date: 197907

        Title: 7.5 minute Digital Elevation Models

        Geospatial_Data_Presentation_Form: map

        Publication_Information:

          Publication_Place: Reston, VA

          Publisher: U. S. Geological Survey

    Description:

      Abstract:

        Digital Elevation Model (DEM) is the terminology adopted by

        the USGS to describe terrain elevation data sets in a digital

        raster form.  The standard DEM consists of a regular array of

        elevations cast on a designated coordinate projection system.

        The DEM data are stored as a series of profiles in which the

        spacing of the elevations along and between each profile is in

        regular whole number intervals.  The normal orientation of

        data is by columns and rows.  Each column contains a series of

        elevations ordered from south to north with the order of the

        columns from west to east.  The DEM is formatted as one

        ASCII header record (A- record), followed by a series of profile

        records (B- records) each of which include a short B-record

        header followed by a series of ASCII integer elevations per each

        profile. The last physical record of the DEM is an accuracy

        record (C-record).



        7.5-minute DEM (30- by 30-m data spacing, cast on Universal

        Transverse Mercator (UTM) projection).  Provides coverage in

        7.5- by 7.5-minute blocks.  Each product provides the same

        coverage as a standard USGS 7.5-minute quadrangle without

        over edge. Coverage is for the Contiguous United States,

        Hawaii, and Puerto Rico.



      Purpose:

        DEM's can be used as source data for digital orthophotos, and, as

        layers in geographic information systems, for earth science

        analysis. DEM's can also serve as tools for volumetric analysis, for

        site location of towers, or for drainage basin delineation.  These

        data were collected as part of the National Mapping Program.



      Supplemental_Information:

        7.5-minute DEMs have rows and columns which vary in length

        and are staggered. The UTM bounding coordinates form a

        quadrilateral (no two sides are parallel to each other), rather

        than a rectangle.  The user will need to pad out the uneven

        rows and columns with blanks or flagged data values, if a

        rectangle is required for the user's application.  Some software

        vendors have incorporated this function into their software

        for input of standard formatted USGS DEMs.

    Time_Period_of_Content:

      Time_Period_Information:

        Range_of_Dates/Times:

          Beginning_Date: 197907

          Ending_Date: present

      Currentness_Reference: ground condition

    Status:

      Progress: In work

      Maintenance_and_Update_Frequency: Irregular

    Spatial_Domain:

      Bounding_Coordinates:

        West_Bounding_Coordinate: -124.7333

        East_Bounding_Coordinate: -067.9500

        North_Bounding_Coordinate: 49.3833

        South_Bounding_Coordinate: 24.5333

    Keywords:

      Theme:

        Theme_Keyword_Thesaurus: none

        Theme_Keyword: DEM

        Theme_Keyword: digital elevation model

        Theme_Keyword: digital terrain model

        Theme_Keyword: hypsography

        Theme_Keyword: altitude

        Theme_Keyword: height

        Theme_Keyword: contour line

        Theme_Keyword: digital contours

      Place:

        Place_Keyword_Thesaurus:

          U.S. Department of Commerce, 1977, Countries,  dependencies,

          areas of special sovereignty, and their principal administrative divisions

          (Federal Information Processing Standard 10-3):Washington, D.C.,

          National Institute of Standards and Technology.

        Place_Keyword: US

        Place_Keyword: CA

        Place_Keyword: MX

        Place_Keyword_Thesaurus:

          U.S. Department of Commerce, 1987, Codes for the identification

          of the States, the District of Columbia  and the outlying areas of

          The United States, and associated areas

          (Federal Information Processing Standard 5-2): Washington, D. C.,

          National Institute of Standards and Technology.

        Place_Keyword: FIPS code of State or Province

        Place_Keyword_Thesaurus:

          U.S. Department of Commerce,1990, Counties and equivalent

          entities of The United States, its possessions, and associated areas

          (Federal  Information Processing Standard 6-4):  Washington, D.C.

          National Institute of Standards and Technology.

        Place_Keyword: FIPS code for county or counties.

    Access_Constraints: None

    Use_Constraints:

      None.  Acknowledgement of the U.S. Geological Survey would be

      appreciated         in products derived from these data.

Data Quality Information

  Data_Quality_Information:

    Attribute_Accuracy:

      Attribute_Accuracy_Report:

        The accuracy of a DEM is dependent upon the level of

        detail of the source and the grid spacing used to sample

        that source. The primary limiting factor for the level of

        detail of the source is the scale of the source materials.

        The proper selection of grid spacing determines the level

        of content thatmay be extracted from a given source

        during digitization.

    Logical_Consistency_Report:

      The fidelity of the relationships encoded in the data

      structure of the DEM are automatically verified using

      a USGS software program upon completion of the data

      production cycle. The test verifies full compliance to the

      DEM specification.

    Completeness_Report:

      The DEM is visually inspected for completeness on a DEM view

      and edit system for the purpose of performing a final quality

      control and if necessary edit of the DEM. The physical format

      of each digital elevation model is validated for content

      completeness and logical consistency during production quality

      control and prior to archiving in the National Digital

      Cartographic Data Base.

      Due to the variable orientation of the quadrilateral in relation

      to the Universal Transverse Mercator (UTM) projection grid,

      profiles that pass within the bounds of the DEM quadrilateral,

      may be void of elevation grid points, and are not represented

      in the DEM. This condition occurs infrequently and is always the

      first or last profile of the dataset.

      Level 2 DEM: Level 2 DEM's may contain void areas due to

      interruptions to contours in the source graphic or DLG. Void

      area elevation grid posts are assigned the value of -32,767. In

      addition, suspect elevation areas may exist in the DEM but are

      not specifically identified. Suspect areas can be located on the

      source graphic as a "disturbed surface, " symbolized by

      contours overprinted with photorevised or other surface

      patterns.

    Positional_Accuracy:

      Horizontal_Positional_Accuracy:

        Horizontal_Positional_Accuracy_Report:

          The horizontal accuracy of the DEM is expressed

          as an estimated root mean square error (RMSE).

          The estimate of the RMSE is based upon

          horizontal accuracy tests of the DEM source

          materials which are selected as equal to or less

          than intended horizontal RMSE error of the DEM.

          The testing of horizontal accuracy of the source

          materials isaccomplished by comparing the

          planimetric (X and Y) coordinates of well-defined

          ground points with the coordinates of the same

          points as determined from a source of higher

          accuracy.

        Quantitative_Horizontal_Positional_Accuracy_Assessment:

          Horizontal_Positional_Accuracy_Value: RMSE of the DEM.

          Horizontal_Positional_Accuracy_Explanation:

            Digital elevation models meet horizontal National Map

            Accuracy Standards (NMAS) accuracy requirements.

      Vertical_Positional_Accuracy:

        Vertical_Positional_Accuracy_Report:

          The vertical RMSE statistic is used to describe the

          vertical accuracy of a DEM, encompassing both

          random and systematic errors introduced during

          production of the data. The RMSE is encoded in

          element number 5 of record C of the DEM.

          Accuracy is computed by a comparison of linear

          interpolated elevations in the DEM with

          corresponding known elevations. Test points are

          well distributed, representative of the terrain, and

          have true elevations with accuracies well within

          the DEM accuracy criteria.  Acceptable test points

          include, in order of preference: field control,

          aerotriangulated test points, spot elevations, or

          points on contours from existing source maps

          with appropriate contour interval.  A minimum of

          28 test points per DEM is required to compute the

          RMSE, which is composed of a single test using 20

          interior points and 8 edge points. Edge points are

          those which are located along, at, or near the

          quadrangle neatlines and are deemed by the

          editor to be useful to evaluating the accuracy of

          the edge of the DEM. Collection of test point data

          and comparison of the DEM with the quadrangle

          hypsography are conducted by the quality control

          units within the USGS.



          There are three types of DEM vertical errors;

          blunder, systematic and random.  These errors

          are reduced in magnitude by editing but cannot be

          completely eliminated. Blunder errors are those

          errors of major proportions and are easily

          identified and removed during interactive editing.

          Systematic errors are those errors that follow

          some fixed pattern and are introduced by data

          collection procedures and systems. These error

          artifacts include: vertical elevation shifts,

          misinterpretation of terrain surface due to trees,

          buildings and shadows, and fictitious ridges, tops,

          benches or striations.  Random errors result from

          unknown or accidental causes.



          DEM's are edited to correctly depict elevation

          surfaces that correspond to water bodies of

          specified size.



          Level 1 DEM: A RMSE of 7-meters or less is the

          desired accuracy standard. A RMSE of 15-meters

          is the maximum permitted. A 7.5-minute DEM at

          this level has an absolute elevation error

          tolerance of 50 meters (approximately three

          times the 15-meter RMSE) for blunder errors for

          any grid node when compared to the true

          elevation.  Any array of points in the DEM can not

          encompass more than 49 contiguous elevations

          in error by more than 21 meters (three times the

          7-meter RMSE).  Systematic errors that are

          within stated accuracy standards are tolerated.



          Level 2 DEM:  A vertical RMSE of one-half of the

          contour interval, determined by the source map,

          is the maximum permitted.  Systematic errors

          may not exceed one contour interval,

          determined by the source map, is the maximum

          permitted.  Systematic errors may not exceed

          one contour interval specified by the source

          graphic.  Level 2 DEMs have been processed or

          smoothed for consistency and edited to remove

          identifiable systematic errors.

        Quantitative_Vertical_Positional_Accuracy_Assessment:

          Vertical_Positional_Accuracy_Value:

          Vertical_Positional_Accuracy_Explanation: RMSE of the DEM.



    Lineage:

      Source_Information:

        Source_Citation:

          Citation_Information:

            Originator: U.S. Geological Survey

            Publication_Date:

            Title: digital contour lines

            Geospatial_Data_Presentation_Form: map

            Publication_Information:

              Publication_Place: Reston, VA

              Publisher: U.S. Geological Survey

        Type_of_Source_Media: magnetic tape

        Source_Time_Period_of_Content:

          Time_Period_Information:

            Range_of_Dates/Times:

              Beginning_Date: 197907

              Ending_Date: present

          Source_Currentness_Reference: ground condition

        Source_Citation_Abbreviation: CONTOUR1

        Source_Contribution:

          hypsographic vector information which is interpolated to regular

          grid posts to form DEM grids in 30- by 30- meter UTM data spacing

          within the 7.5 minute DEM bounds.



      Source_Information:

        Source_Citation:

          Citation_Information:

            Originator: U.S. Geological Survey

            Publication_Date:

            Title: photo ID number

            Geospatial_Data_Presentation_Form: remote-sensing image

            Publication_Information:

              Publication_Place: Reston, VA

              Publisher: U.S. Geological Survey

        Type_of_Source_Media: transparency

        Source_Time_Period_of_Content:

          Time_Period_Information:

            Range_of_Dates/Times:

              Beginning_Date: various

              Ending_Date: various

          Source_Currentness_Reference: ground condition

        Source_Citation_Abbreviation: PHOTO1

        Source_Contribution: elevation values



      Source_Information:

        Source_Citation:

          Citation_Information:

            Originator: U.S. Geological Survey

            Publication_Date: Unpublished material

            Title: project control

            Geospatial_Data_Presentation_Form: map

            Publication_Information:

              Publication_Place: Reston, VA

              Publisher: U.S. Geological Survey

        Type_of_Source_Media: magnetic tape

        Source_Time_Period_of_Content:

          Time_Period_Information:

            Range_of_Dates/Times:

              Beginning_Date: various

              Ending_Date: various

          Source_Currentness_Reference: ground condition

        Source_Citation_Abbreviation: CONTROL1

        Source_Contribution: ground control points



      Process_Step:

        Process_Description:

          The production procedures, instrumentation,

          hardware and software used in the collection of

          standard U. S. Geological Survey (USGS) Digital

          Elevation Models (DEM's) vary depending on

          systems used at the contractor, cooperator or

          National Mapping Division (NMD) production sites.

          This process step describes, in general, the

          process used in the production of standard USGS

          DEM datasets.



          Level 1 DEM: Level 1 DEM's are acquired

          photogrammetrically by manual profiling or image

          correlation techniques from National Aerial

          Photography Program (NAPP) or equivalent

          source photographs. Level 1 30-minute DEM's

          may be derived or resampled from level 1 7.5-

          minute DEM's.



          Level 2 DEM: Level 2 DEM's are produced by

          converting 1:24,000-scale and 1:100,000-scale

          hypsography digital line graph (DLG) data to DEM

          format or the DEM's are generated from vector

          data derived from scanned raster files of USGS

          1:24.000-scale or 1:100,000-scale map series

          contour separates.



          Level 3 DEM: Level 3 DEM's are created from DLG

          data that has been vertically integrated with all

          categories of hypsography, hydrography, ridge

          line, break line, drain files and all vertical and

          horizontal control networks. The production of

          level 3 DEMs requires a system of logic

          incorporated into the software interpolation

          algorithms that clearly differentiates and

          correctly interpolates between the various types

          of terrain, data densities and data distribution.



          Water body editing: DEM surface areas

          corresponding to water bodies are flattened and

          assigned map specified or estimated surface

          elevations.  Water body areas are defined as

          ponds, lakes, and reservoirs that exceed 0.5

          inches at map scale and double line drainage that

          exceeds 0.25 inches at map scale. Water body

          shorelines are derived either from a hypsographic

          DLG or by interactive delineation from 1:24,000-

          scale or 1:100,000-scale USGS map series.



          Edge matching:  DEM datasets within a project area 

          (consisting of a number of adjacent files) are edge 

          matched to assure terrain surface continuity

          between files.  Edge matching is the process of

          correcting adjacent elevation values along common edges. 

          The objective of edge matching is to create more accurate 

          terrain representations by correcting the alignment of 

          ridges and drains, and overall topographic shaping within 

          an approximately 25-30 row or column grid post

          zone on both edges.



          Quality control:  DEM's are viewed on interactive

          editing systems to identify and correct blunder

          and systematic errors.  DEM's are verified for

          physical format and logical consistency at the

          production centers and before archiving in the

          National Digital Cartographic Data Base (NDCDB)

          utilizing the Digital Elevation Model Verification

          System (DVS) software.

        Source_Used_Citation_Abbreviation: CONTOUR1, PHOTO1, CONTROL1

        Process_Date:

Spatial Data Organization Information

  Spatial_Data_Organization_Information:

    Direct_Spatial_Reference_Method: raster


    Raster_Object_Information:

      Raster_Object_Type: grid cell

      Row_Count:

      Column_Count:

Spatial Reference Information

  Spatial_Reference_Information:

    Horizontal_Coordinate_System_Definition:

      Planar:

        Grid_Coordinate_System:

          Grid_Coordinate_System_Name: Universal Transverse Mercator

          Universal_Transverse_Mercator:

            UTM_Zone_Number: 10-19

            Transverse_Mercator:

              Scale_Factor_at_Central_Meridian: .9996

              Longitude_of_Central_Meridian: depends on zone

              Latitude_of_Projection_Origin: 0.0

              False_Easting: 500000

              False_Northing: 0.0

        Planar_Coordinate_Information:

          Planar_Coordinate_Encoding_Method: row and column

          Coordinate_Representation:

            Abscissa_Resolution: 30

            Ordinate_Resolution: 30

          Planar_Distance_Units: meters

      Geodetic_Model:

        Horizontal_Datum_Name: North American Datum 1927

        Ellipsoid_Name: Clark 1866

        Semi-major_Axis: 6378206.4

        Denominator_of_Flattening_Ratio: 294.9787

    Vertical_Coordinate_System_Definition:

      Altitude_System_Definition:

        Altitude_Datum_Name: National Geodetic Vertical Datum of 1929

        Altitude_Resolution: 1

        Altitude_Distance_Units: feet or meters

        Altitude_Encoding_Method: explicit elevation coordinate included with horizontal coordinates

Entity and Attribute Information

  Entity_and_Attribute_Information:

    Overview_Description:

      Entity_and_Attribute_Overview:

        The digital elevation model is composed of a 6-

        character integer raster representing a gridded form of

        a topographic map hypsography overlay. Each raster

        entity contains a 6-character integer value between

        -32,767 to 32,768.



      Entity_and_Attribute_Detail_Citation:

        U.S.Department of the Interior, U.S. Geological Survey, 1992,

        Standards for digital elevation models: Reston, VA,



        A hypertext version is available at:

         URL ftp://nmdpow9.er.usgs.gov/public/dem_html/standards_dem.html



        Softcopy in ASCII format is available at:

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt1.txt 

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt2.txt 

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt3.txt 



        Softcopy in WordPerfect format is available at:

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt1.wp5 

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt2.wp5 

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt3.wp5 



        Softcopy in PostScript format is available at:

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt1.ps 

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt2.ps 

         URL ftp://nmdpow9.er.usgs.gov/public/demstnds/stdempt3.ps 

Distribution Information

  Distribution_Information:

    Distributor:

      Contact_Information:

        Contact_Organization_Primary:

          Contact_Organization: Earth Science Information Center, U.S. Geological Survey

        Contact_Address:

          Address_Type: mailing address

          Address: 507 National Center

          City: Reston

          State_or_Province: Virginia

          Postal_Code: 22092

        Contact_Voice_Telephone: 1 800 USA MAPS

        Hours_of_Service: 0800-1600

        Contact_Instructions:

          In addition to the address above there are  other ESIC offices

          throughout the country.  A full list of these offices is at:

           URL http://www-nmd.usgs.gov/esic/esic_index.html 

    Resource_Description: 7.5-minute digital elevation models

    Distribution_Liability:

      Although these data have been processed successfully on a

      computer system at the U.S. Geological Survey, no warranty

      expressed or implied is made by the Geological Survey

      regarding the utility of the data on any other system, nor shall

      the act of distribution constitute any such warranty. USGS will

      warrant the delivery of this product in computer-readable

      format. and will offer appropriate adjustment of credit when

      the  product is determined unreadable by correctly adjusted

      computer input peripherals, or when the physical medium is

      delivered in damaged condition. Requests for adjustments of

      credit must be made within 90 days from the date of this

      shipment from the ordering site.



    Standard_Order_Process:

      Digital_Form:

        Digital_Transfer_Information:

          Format_Name: DEM

          Format_Information_Content:

            USGS standard DEM: The standard USGS

            DEM can be described as an ASCII formatted

            elevation file preceded by a metadata

            header file which consists of one 1024 byte

            ACSII record.

          Transfer_Size: 1

        Digital_Transfer_Option:

          Offline_Option:

            Offline_Media: 9-track tape or  8-mm cartridge tape

            Recording_Format:

              standard 9-track label, record length 1024.  To ensure

              compatibility, a maximum blocking factor (specified in

              multiples of 1024) of 31744 is recommended.  For efficiency,

              blocking factors less than 16,000 are discouraged.

            Compatibility_Information: none

      Fees:

        The online copy of the data set (when available electronically) may

        be accessed without charge. For 8-mm cartridge and 9-track tapes the

        costs are:

        1 digital product = $40

        2 digital products = $60

        3 digital products = $80

        4 digital products = $100

        5 digital products = $120

        6 or more = $90 plus $7 per each product over six

Metadata Reference Information

  Metadata_Reference_Information:

    Metadata_Date: 199501

    Metadata_Contact:

      Contact_Information:

        Contact_Organization_Primary:

          Contact_Organization: U.S. Geological Survey

        Contact_Address:

          Address_Type: mailing address

          Address: 508 National Center

          City: Reston

          State_or_Province: VA

          Postal_Code: 22092

        Contact_Voice_Telephone: 703 648 4543

    Metadata_Standard_Name: Content Standards for Digital Geospatial Metadata

    Metadata_Standard_Version: 19940608






7.5 minute Digital Elevation Models (lattices)





Identification_Information

Data_Quality_Information

Spatial_Data_Organization_Information

Spatial_Reference_Information

Entity_and_Attribute_Information

Distribution_Information

Metadata_Reference_Information






Identification Information

Section Index

 






Data Quality Information

Section Index

 






Spatial Data Organization Information

Section Index

 






Spatial Reference Information

Section Index

 






Entity and Attribute Information

Section Index

 






Distribution Information

Section Index

 






Metadata Reference Information

Section Index

 



SMMS Metadata report generated 10/19/99