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Photogrammetry Digital Elevation Models Orthophotographs

Photogrammetry Digital Elevation Models Orthophotographs. Topographic Mapping – Old School. Surveying Instruments. Stadia Rod Distance and elevation measurement Interval between crosshairs gives distance Elevation on rod gives elevation relative to observer. Stadia Rod.

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Photogrammetry Digital Elevation Models Orthophotographs

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  1. PhotogrammetryDigital Elevation Models Orthophotographs

  2. Topographic Mapping – Old School

  3. Surveying Instruments • Stadia Rod • Distance and elevation measurement • Interval between crosshairs gives distance • Elevation on rod gives elevation relative to observer

  4. Stadia Rod

  5. Surveying Instruments • Transit • Record data in field for later analysis • Alidade • Used for direct plotting in field • Plane Table • Used in conjunction with Alidade • Plot distance and elevation directly on rough map

  6. Laser Ranger

  7. Retroreflector

  8. View Through Sight

  9. Installing Bench Marks

  10. Aerial Photographs • Altitude variation during flight • Camera tilt (Doesn’t always point straight down) • Scale varies across photograph • Scale varies with elevation • Foreshortening toward edge of picture • Parallax shift with altitude • Lens distortion • Atmospheric distortion

  11. Photogrammetry • Use overlap of aerial photos to view photos in stereo • Correct photos for camera angle and altitude • Parallax shift determines altitude

  12. KelshStereoplotter: Concept

  13. KelshStereoplotter in Practice

  14. Analog Stereoplotter

  15. Analytical Stereoplotter • One step below complete automation • Photos scanned digitally • Digital images corrected for camera angle and altitude • Operator views images through a stereoviewer • Joystick used to maneuver • Results stored directly as digital file

  16. Digital Photogrammetry • Not feasible until 1980’s when computers had sufficient speed and memory • Match features on photos by recognition routines • Determine parallax and calculate x,y,z

  17. Digital Elevation Models • Derive from existing maps and survey data • Derive from radar or laser ranging • All field-derived data are irregular • Need to generate grid of points • Need DEM’s to generate modern orthophotographs • DEM coverage of Mars and Venus is as good as Earth

  18. Orthophotographs • Aerial photographs with parallax and scale distortions removed • Analog methods are modified from photogrammetry • Instead of scribing a contour line, expose a patch of the images on film • First invented by Germans and French in 1930’s • Russell Bean of USGS invented a method in 1955 • Systematic production began at USGS in 1965 • Analog methods used until early 1980’s

  19. Digital Orthophotographs • Begun in 1980’s when computers finally had enough memory and speed to handle the load • USDA wanted base maps for soil mapping; contributed funding for development and production • Full scale production began in 1991, peaked in 1999 • Now nearly complete • DOQ = Digital Orthophoto Quadrangle

  20. How Orthophotos are Made • Computer recognizes locations on photograph • Control points on ground for location accuracy • Elevation provided by DEM • Not entirely like digital photogrammetry • Image generally overlaps latitude-longitude bounds by 50-300 meters • All use NAD 83 and Universal Transverse Mercator projection

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