February 11, 2000, the Space Shuttle gathered topographi

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Chad Schaeding Surveying EngineeringFerris
State UniversityBig Rapids, Michigan
National Digital Elevation Models MSPS Conference
Grand Rapids Michigan, USA February 17, 2006
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Presentation outline

• Introduction to National Digital Elevation Models
• National Elevation Data Set (NED)
• Shuttle Radar Topography Mission (SRTM)
• FREE! From the United States Geological Survey
• Accuracy Analysis Comply w/Mapping Standards
• Applications
• Differentially Rectify Imagery (Orthoimage)
• Conclusions

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National Elevation Data Set (NED)

• The National Elevation Dataset is the new format
  for existing U.S. Geological Survey (USGS)
  elevation data
• The development of NED began in the early 1990s
  and was completely assembled in 1999
• Processing individual 7.5 minute 124,000-scale
  (163,360 for Alaska) DEMs
• Datum Conversions to NAD83/NAVD88
• Projection Transformation and Resampling
• Artifact Correction
• Edge Matching and Merging with adjacent DEMs
• Seamless form with a consistent datum, elevation
  unit, projection, at 10 and 30 meter resolution
  (30 and 70 respectively)

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Photogrammetric Data Acquisition

• Most USGS DEMs were collected using stereo image
  pairs adjacent photographs with overlap
• High Altitude Aerial Photography Program

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Shuttle Radar Topography Mission

• February 11, 2000, the Space Shuttle gathered
  topographic data over approximately 80 of the
  land surfaces of the Earth in only 11 days.

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SRTM

• Acquired by the National Geospatial-Intelligence
  Agency (NGA) and the National Aeronautics and
  Space Administration (NASA) utilizing a radar
  system that flew onboard the Space Shuttle
  Endeavour
• SRTM DEM is available with WGS84 datum and EGM96
  geoid model
• Currently two products are available
• One arc-second resolution (90) for the United
  States and its territories
• Three arc-second (270) for all the areas
  between 60º North and 56º South latitudes   

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What is Radio Detection and Ranging (RADAR) 

• Definition A method, system or technique, for
  using beamed, reflected, and timed
  electromagnetic radiation to detect, locate, and
  (or) track objects, to measure altitude and to
  acquire a terrain image.

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Interferometric Synthetic Aperture Radar

• SAR interferometry is a technique involving phase
  measurements from successive aircraft or
  satellite SAR images to infer differential range
  and range changes for the purpose of detecting
  very subtle changes on or of the earths surface
  with unprecedented scale, accuracy, and
  reliability.

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http//seamless.usgs.gov/
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Accuracy Analysis of NED and SRTM

• NED SRTM

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Previous Research of NED and SRTM

• Smith and Sandwell (2003) performed spectral
  analysis of the 1-arcsecond SRTM and NED data and
  found that following
• Root Mean Squared (RMS) of SRTM data is 2.7m
• RMS of the NED data 3.5m  
• Reinartz et al (2005) conclude that SRTM data
  accuracy decreases drastically in forest areas
  since it neither represents the tree canopy or
  the ground.

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National Map Accuracy Standard

• The NMAS defines the following two criteria to
  test the vertical accuracy of a topographic map
• Vertical accuracy, as applied to contour maps on
  all publication scales, shall be such that not
  more than 10 percent of the elevations tested
  shall be in error by more than one-half the
  contour interval 
• The accuracy of any map may be tested by
  comparing the positions of points whose locations
  or elevations are shown upon it with
  corresponding positions as determined by surveys
  of a higher accuracy

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American Society for Photogrammetry and Remote
Sensing

• The Root-Mean-Square Error (RMSE) statistic is
  used to evaluate the accuracy of spatial data is
• Class 1 map should have a vertical RMSE of 1/3
  the contour interval for well-defined points and
  1/6 the contour interval for spot elevations.
• Maps compiled within limiting RMSE errors of
  twice or three times those allowed for Class 1
  map shall be designated as Class 2 or Class 3,
  respectively.

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Case Study, FSU Golf Course

• Evaluating the accuracy of SRTM/NED data

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Case Study, FSU Golf Course

• Even though USGS standards for DEM require only
  28 check points with at least eight scattered
  around the edge
• More than 500 points were collected in Real-time
  Kinematic (RTK) mode
• Base Station is the Big Rapids Continuous
  Operating Reference Station (CORS) at a distance
  of less than 1 mile
• Visual inspection on the elevation data and
  products compared to more accurate GPS data

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Case Study, Elevation Results (m)
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Case Study, Slope Results ()
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Orthophotographs

• Orthophotography is a geometrically corrected
  photograph created from either aerial or
  satellite imagery
• The most expensive part of producing an
  orhtophoto is generally the creation of the DEM

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Case study, the FSU golf course

• Two orthophotographs were created using the Leica
  Photogrammetry Suite from 110,000-scale
  photography taken at a flight height of 1,582
  meter above the average terrain and scanned at
  ground resolution of 0.15 meters
• The initial NED and SRTM DEMs were projected from
  their native geographic coordinates to Michigan
  Plane coordinate system to create a 35x35 meter
  resolution DEM

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Concluding Remarks

• From the results of experiments undertaken in
  this study, it is clear that these government
  datasets can be used to create orthophotos at a
  scale of 110,000 that meet acceptable industry
  standards such as those developed by ASPRS.
• This study found that the SRTM data had slightly
  better accuracy than the NED data but it may not
  represent the terrain properly and may have
  larger errors in computing slope and aspect
  parameters. It is also important to note that the
  SRTM data is a DSM while NED data is a DEM
  measuring ground topography.
• SRTM data is current which is an important
  advantage providing a proper model that can be
  used for many applications, even for updating the
  NED.

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Questions
The support for this research from the National
Geospatial-Intelligence Agency under contract no.
HM1582-04-1-2026 is greatly acknowledged.