Digital Elevation Model (USGS DEM), Louisiana, UTM 15 NAD83, Louisiana FEMA Project - Phase 1: Amite River Basin, USACE (2001)

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Frequently-anticipated questions:

What does this data set describe?

    Digital Elevation Model (USGS DEM), Louisiana, UTM 15 NAD83, Louisiana FEMA Project - Phase 1: Amite River Basin, USACE (2001)
    Digital Elevation Model (DEM) of Louisiana. The data were derived from LIDAR measurements performed over a period of years beginning in 1999, and are presented at a elevation posting interval of five meters. The model is in USGS (non-SDTS) DEM format.

    These data were produced for the Louisiana Federal Emergency Management Agency (FEMA) Project - Phase 1: Amite River Basin, Louisiana under the St. Louis District U.S. Army Corps of Engineers (USACE) contract number ACW43-98-D-0517.

    This metadata record serves as a master document for each individual data record currently available for download. Data are distributed and available online in quarter-quadrangles. Each downloadable quarter-quadrangle comes complete with metadata specific to that location.

  1. How should this data set be cited?

    U. S. Army Corps of Engineers, Saint Louis District, 2001, Digital Elevation Model (USGS DEM), Louisiana, UTM 15 NAD83, Louisiana FEMA Project - Phase 1: Amite River Basin, USACE (2001).

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -91.5
    East_Bounding_Coordinate: -89.8
    North_Bounding_Coordinate: +31.00
    South_Bounding_Coordinate: +30.00

  3. What does it look like?

    DEM_UTM15_NAD83_phase1_USACE.gif (GIF)

  4. Does the data set describe conditions during a particular time period?

    Calendar_Date: Mar-1999
    Approximate date of LIDAR survey used to build the source data for this data set

  5. What is the general form of this data set?

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Raster data set. It contains the following raster data types:

      • Dimensions, type Point

    2. What coordinate system is used to represent geographic features?

      Grid_Coordinate_System_Name: Universal Transverse Mercator
      UTM_Zone_Number: 15
      Scale_Factor_at_Central_Meridian: 0.9996
      Longitude_of_Central_Meridian: -93.000000
      Latitude_of_Projection_Origin: +00.000000
      False_Easting: 500000
      False_Northing: 0

      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest 5
      Ordinates (y-coordinates) are specified to the nearest 5
      Planar coordinates are specified in meters

      The horizontal datum used is North American Datum of 1983.
      The ellipsoid used is Geodetic Reference System 80.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257.

      Altitude_Datum_Name: North American Vertical Datum of 1988
      Altitude_Resolution: 0.01
      Altitude_Distance_Units: Survey Feet
      Explicit elevation coordinate included with horizontal coordinates

  7. How does the data set describe geographic features?

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

    FEMA, USGS, 3001, Inc., LOSCO

  3. To whom should users address questions about the data?

    U.S. Army Corps of Engineers, Saint Louis District
    c/o Bob Mesko
    1222 Spruce Street
    Saint Louis, Missouri 63103-2833

    (314) 331-8746 (voice)

Why was the data set created?

DEM data are useful for terrain analysis and modeling including slope and aspect calculations. They may be used to produced shaded relief maps and contour maps.

How was the data set created?

  1. From what previous works were the data drawn?

    (source 1 of 1)
    U. S. Army Corps of Engineers, Saint Louis District, 2001, Digital Elevation Model (ArcInfo lattice), Louisiana, UTM 15 NAD83, Louisiana FEMA Project - Phase 1: Amite River Basin, USACE (2001).

    Type_of_Source_Media: computer file
    Source_Contribution: Regular lattice of elevation values

  2. How were the data generated, processed, and modified?

    Date: 2001 (process 1 of 1)
    ArcInfo Lattice was processed with ArcInfo LATTICEDEM to produce a USGS format DEM.

    Person who carried out this activity:

    3001, Inc.
    LIDAR Manager
    5525 Mounes, Suite 102
    New Orleans, LA 70123

    (504) 733-3001 (voice)

  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    The DEM has been clipped to the area of the 3.75 minute cell comprising the SE of the Pride. All lattice points outside this area that comprise the balance of the minimum bounding rectangular area in UTM coordinates have the 'no data' elevation value of -32767.

  5. How consistent are the relationships among the observations, including topology?

    The DEM was produced with an origin at an even increment of the DEM point spacing to allow adjacent tiles to be appended without resampling.

    One of the primary goals of this project was to produce higher accuracy elevation data than was available for the Amite River Basin. Because of the wide array of uses for the data products, many standards had to be taken into consideration for data production. Among these were the FEMA standard for LIDAR capture and data processing, and the USGS standard for Digital Elevation Models. With regard to the USGS standard for Digital Elevation Models, the resolution of the data produced in this project is higher than that for which the standard guidelines were developed. Although this was the overriding goal of the project, it also requires the data follow the standard more loosely than if the data were spaced at the same intervals and resolution as regulated by the standard.

    The USGS standard provides guidelines for 5 primary types of DEMs. The main difference between the types is the geographic coverage of the data and the resolution of GRID values within the DEM. The smallest geographic extent and grid spacing (and thus the most accurate DEM) documented in the standard is the 7.5 minute DEM (which is a typical USGS Quadrangle). The standard states that the resolution of the 7.5 minute DEM âwill have a 10 or 30 meter grid spacingâ. The DEMs produced in this project are tiled to USGS Quarter Quadrangles (or ¼ of the typical 7.5 minute quad). The GRID spacing is 5-meters, as opposed to the 10 or 30 meter requirement of the standard.

    It should also be noted that when these DEMs are viewed with the USGS DLG Viewer software, there are inconsistencies in the elevation data reported by the viewer.

    These inconsistencies have not been observed in other viewers, including ArcInfo, ArcView, ERDAS and Terramodel. The reason for the inconsistencies is that the DEM generated by ArcInfo utilizes data elements of the USGS standard that DLG Viewer does not take into account when computing elevations.

    Based on the USGS "Standards for Digital Elevation Models", a DEM file is organized into a series of three records, A, B, and C. The A record contains information defining the general characteristics of the DEM, including its name, boundaries, units of measurement, minimum and maximum elevations, number of B records, and projection parameters. Each B record consists of an elevation profile with associated header information, and the C record contains accuracy data. Each file contains a single A and C record, while there is a separate B record for each elevation profile.

    Two data elements in particular appear to cause the rendering inconsistencies with the DLG Viewer software. The data elements are the z resolution (data element 15, record b) and the local datum for the profiles (data element 4, record b). The DLG viewer appears to ignore one of the math expressions or uses a default value to compute the elevations. The other viewers listed above can do the math and show the z-values properly. In these viewers, elevations are computed by multiplying the z-values by the z resolution and then adding the local datum value. By either using a default value for the z resolution or datum, or not taking one of these elements into consideration when computing the elevation, the DGL Viewer is not displaying the correct elevation.

    According to the USGS "Standards for Digital Elevation Models" the z resolution does not have to be a multiple of 10 (e.g 1, .1, .01, etc.). This is the case for the DEMs created by ArcInfo for this project (e.g. .286508). As far as the local datum for the profiles, ArcInfo uses the lowest elevation in the file. According to the USGS Standard, the local datum does not have to be set to zero.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints: none

  1. Who distributes the data set? (Distributor 1 of 1)

    ATLAS - Louisiana GIS Online
    c/o Farrell Jones
    Baton Rogue, Louisiana 70803

  2. What's the catalog number I need to order this data set?

    LiDAR Data

  3. What legal disclaimers am I supposed to read?

    Distributor assumes no liability for the content, accuracy, or use of this data.

  4. How can I download or order the data?

  5. What hardware or software do I need in order to use the data set?

    The USGS DEM (non-STDS) data format will generally require a special viewer/plug-in for use in commercially available GIS software applications. Additionally, all data are distributed in ZIP compression format, which will require a decompresssion and extraction application in order for the data to be utilized.

Who wrote the metadata?

Last modified: 05-Dec-2002
Last Reviewed: as needed

Metadata author:
LA Geographic Information Center
E302 Howe-Russell
Baton Rouge, LA 70803

Hours_of_Service: 8 am - 4:30 pm M-F
Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)

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LA Geographic Information Center (LAGIC) (C), 2002
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