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Photogrammetry Digital Elevation Models Orthophotographs.

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Presentation on theme: "Photogrammetry Digital Elevation Models Orthophotographs."— Presentation transcript:

1 Photogrammetry Digital 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 (Doesnt 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 Kelsh Stereoplotter: Concept

13 Kelsh Stereoplotter 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 1980s 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 DEMs 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 1930s – Russell Bean of USGS invented a method in 1955 – Systematic production began at USGS in 1965 – Analog methods used until early 1980s

19 Digital Orthophotographs Begun in 1980s 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 meters All use NAD 83 and Universal Transverse Mercator projection

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