Introduction to Digital Elevation Models

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Introduction to Digital Elevation Models Powered By Docstoc
					   Digital Elevation Models
   GLY 560:
     GIS and Remote Sensing for
        Earth Scientists


Class Home Page:
http://www.geology.buffalo.edu/courses/gly560/
Creation of DEM’s
  • Conversion of contour lines
  • Photogrammetry
  • Satellite Stereo
  • Radar Stereo
  • Radar Inferometry
  • Laser Altimetry
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Conversion of printed contour lines
  • Existing map plates are scanned
  • Resulting raster is vectorized and edited
    contours are "tagged" with elevations
  • Additional elevation data are created from the
    hydrography layer (e.g. shorelines provide
    additional contours)
  • Algorithm is used to interpolate elevations at
    every grid point from the contour data


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Photogrammetry
  • Manually: an operator looks at a pair of
    stereophotos through a stereoplotter and must
    move two dots together until they appear to be
    one lying just at the surface of the ground
  • Automatically: an instrument calculates the
    parallax displacement of a large number of
    points (e.g. for USGS 7.5 minute quadrangles,
    the Gestalt Photo Mapper II correlates 500,000
    points)
  • Correction of elevation from photographs: water
    bodies are assumed to be flat.

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Stereo Satellite


   • Two satellite
     passes are
     combined to get
     effective “stereo”
     view




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Stereo Radar
  • Works like
    photogrammetry but
    use radar wave
    instead.
  • Can be done from
    space or airborne with
    side-looking-airborne-
    radar (SLAR)
  • Can penetrate
    vegetation canopy

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Radar Interferometry
  • Use phase
    difference in two
    radar signals to
    measure elevation
    differences
  • Signals are from
    two receivers so
    called “synthetic
    aperture radar” or
    SAR.

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Laser Altimetry
  • Fly laser over area, and time reflection of
    laser.




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Available Resolutions of DEM




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7.5-minute USGS DEM
  • Resolution:
    10 m or 30 m
  • Coverage:
    7.5 x 7.5 min Quad (1:24,000)
  • Vertical Accuracy:
    7m


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10m and 30m DEM Availability




http://mcmcweb.er.usgs.gov/status/dem_stat.html


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7.5-minute USGS DEM Data
  Classification Levels:
  • Level-1 Scanned from National High Altitude
    Photography (NHAP)/NAPP photography. A vertical
    RMSE of 7 meters is the desired accuracy standard.
    (Most 7.5” DEMs are Level-1)
  • Level-2 Processed or smoothed for consistency and
    edited to remove systematic errors. A RMSE of 1/2
    contour interval.
  • Level-3 DEMs are derived from DLG data by
    incorporating hypsography (contours, spot elevations)
    and hydrography (lakes, shorelines, drainage). A RMSE
    of 1/3 contour interval.

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30-minute USGS DEM
  • Resolution:
    2 arc seconds (~60m)
  • Coverage:
    30x30 min block (½ 1:100,000)
  • Vertical Accuracy:
    25 m


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1- degree USGS DEM
  • Resolution:
    3 arc seconds (~100m)
  • Coverage:
    1x1 deg block (½ 1:250,000)
  • Vertical Accuracy:
    25 m


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“Seamless DEM” Data
  • Buy DEM from USGS already in mosaic form by
    National Mapping Program




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Global DEMs
  • GTOPO30
  • Aster
  • Shuttle Mapping Program
  • IKONOS




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GTOPO30
  • Resolution:
    30-second (~1 km)
  • Coverage:
    50 deg lat x 40 deg long
  • Vertical Accuracy:
    30 m


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ASTER DEM
  • Advanced Spaceborne Thermal
    Emission and Reflection Radiometer
  • Off-Nadir pointing allows DEM
  • First international DEM of decent
    quality available.
  • Must request generation of DEM
    (slow turnaround) but FREE!
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Aster DEM
  • GTOPO30                          • ASTER
    (1 km2/pixel)                      (30 m2/pixel)




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